Two-Keyed Lock

Sometimes one man can't be trusted so certain locks require two, or more, people to unlock, simultaneously. Maybe it's the keys to the world's destruction, or the kingdom's treasure, but whatever it is it's so important you know it must be good... or really bad. Hence the Two-Keyed Lock.

This is common in military settings, and very much Truth in Television with Nuclear Weapons: The places where the keys are turned are always too far apart from each other for one person to be able to do it alone. This is an implementation of the more general military concept of "Two Person Control" (TPC).

A variation seen in video games, especially ones with co-op play, is a stock puzzle that requires two characters to activate triggers in different places at the same time to complete an objective.

Anime and Manga
"Gendo: Shut down the I/O system. (Hyuga and Aoba insert keys into their respective slots) Hyuga: Three! Two! One! (keys are turned) (beat) Hyuga: WE CAN'T SHUT IT DOWN!!!"
 * Goldion Crusher in GaoGaiGar.
 * In episode 13 of Neon Genesis Evangelion, an Angel infiltrates NERV's computer system and Gendo tries to be Genre Savvy.


 * Then a few minutes later Maya and Ritsuko do this to hack into the Angel through Casper while Casper is being hacked by the Angel in turn. They enter the final command simultaneously and it works, killing the Angel and disarming the MAGI's self-destruct sequence a single second before it could blow the entire base to kingdom come.
 * In fact, this is how the MAGI self-destruct works: the three cores vote among themselves. Starting or cancelling the sequence requires unanimity of all three; cheating is impossible since any attempts at one core hacking another are immediately discovered. If two disagree and the third is undecided, they'll ask the human crew.
 * The Cyclops system in Gundam Seed was so inhumane (and probably expensive, considering that it was one-shot only) that it required at least five top Earth Alliance generals' keys to turn on.
 * It was actually only two in the anime, and the system was set up so that it was possible to have one person to turn both keys as long as he had them. The keys were kept on two separate generals, however.
 * Apparently, the final Power Limiter placed upon Hayate Yagami in Magical Girl Lyrical Nanoha StrikerS had to be removed simultaneously by Admiral Chrono Harlaown and Knight Carim, making it a Two-Keyed Lock even though they didn't have to be physically present near Hayate at the time.
 * The mechanism for detonating KaibaCorp Island in Yu-Gi-Oh!! requires two key cards to activate.
 * In Yu-Gi-Oh! GX, Saiou obtains the controls to a Doomsday Device which requires two keys to use. Before he can use them, however, his original, benevolent personality takes over. Knowing that his control is diminishing, he gives one of the keys to Judai and the other to Edo Phoenix, pleading with both  never to give them back. As he predicted, his evil side returns soon after, and for the rest of the arc, his plan focuses on getting them back.

Comic Books

 * The lab where the extremist samples were kept in Iron Man: Extremis. This was a key plot point.

Film

 * Terminator 2, to access the vault where the arm and chip were kept.
 * WarGames, to control a nuclear launch. One officer gets held at gunpoint for refusing to turn his key when ordered, . That inspires NORAD to turn over launch control to an AI and leads to the main plot of the movie.
 * Played with in Under Siege 2, where it was two passwords on computer keyboards that needed to be entered simultaneously.
 * Superman III, to activate a satellite positioning system; although the two keys in question needed to be inserted simultaneously, not turned.
 * The nuclear missile locks in The Hunt for Red October apparently had one of these. Although we never see the locks themselves, we see Captain Ramius taking possession of both keys—which, as the doctor points out, sort of defeats the point.
 * In the novel, it is stated that five officers are needed to carry out a launch. It also stated that in the event of the political officer's death, the captain was supposed to take charge of his key. There were probably other standing orders as to who inherits the other keys in the event of any of the other officers with launch keys dying on a voyage as well.
 * A two key (two combination in the book) system was also used on a safe in the submarine that contained the mission orders and code books.
 * Used to shut down in Bee Movie.
 * Sunshine has a high-tech version - instead of two keys, to override the autopilot they need the voice patterns from two different crew members.
 * The Librarian has this. "Hey, don't nuclear launch codes require this?" "Who do you think they got it from?"
 * The Lost in Space movie required two keys to activate the hyperdrive.
 * The movie Crimson Tide. Two keys needed to unlock the missile launch controls. The XO refuses the Captain's orders to unlock the controls because they lost communications before the launch order was confirmed as protocol demands. Mutiny ensues.
 * One might notice that this is EXACTLY WHY there ARE two keys.
 * The Captain was in fact acting on legitimate launch orders and the XO was refusing those orders due to the possibility they had received orders to cancel the launch. The real villains of the film were the (unseen) President and Secretary of Defense, who issued nuclear launch orders with the intention of rescinding them if the situation changed. There are no take-backs in thermonuclear warfare.
 * The James Bond film GoldenEye had this for the titular weapon as well.
 * The USS Enterprise self-destruct sequence needed spoken confirmation from three senior officers to trigger in Star Trek III the Search For Spock.
 * That destruct sequence is taken verbatim from the infamously Anvilicious TOS episode, "Let That Be Your Last Battlefield". Twenty years later, they still hadn't changed the codes.
 * In TNG, it's often the Captain and Executive Officer, so the "two senior officers" may be in lieu of the XO in the event that they are unavailable. The Enterprise-E was slated to self-destruct on confirmation from Worf and Crusher since Riker was unavailable at the time.
 * In The First Great Train Robbery, the safes holding the gold need a total of four keys to be opened.
 * In The First Great Train Robbery, the safes holding the gold need a total of four keys to be opened.

Literature

 * The safe deposit vault that Artemis Fowl burgles in The Opal Deception.
 * An escape hatch in Thursday Next: First Among Sequels has two handles which need to be turned simultaneously.
 * In the Gears of War novel, Jacinto's Remnant, Chairman Prescott, Colonel Hoffman and somebody else had to insert three keys and turn simultaneously to activate the Hammer of Dawn technology that destroyed most of the planet. Prescott had it next to his car keys.
 * In the Mass Effect novel, Ascension, Hendel and the one of the quarians had to this to a bomb that is rigged to explode and destroy a ship with a crew of over 500. With the added problem that they couldn't see each other.
 * In The Baroque Cycle, the is locked with three different locks and, later, kept behind two doors which are each locked with three locks. That doesn't stop  from picking them and getting into it.
 * The Lord Darcy story "A Case of Identity" had a two key system for a vault containing the Marquis' official regalia. The door had eight keyholes and two keys. Each man with a key knew which keyhole to use his key in, but not which keyhole the other key went into. Improper timing, or turning a key in the wrong hole, would set off the alarm.
 * In Neuromancer, the artificial intelligence Wintermute can only be freed from its programming constraints if one person speaks a password into a particular compter terminal just as another one breaks through the software defenses.

Live-Action TV

 * One can actually see this decay in Star Trek. In the days of the original series, it takes codes from 3 senior officers to activate the auto-destruct sequence. In the Next-Gen days, it only takes 2 senior officers (and their hand-prints); it also takes both of them to stop the auto-destruct. By the time of Voyager, they fully avert the trope: the ship can be destroyed at the sole command of the captain. Whether this is a good idea...
 * The Xindi planet killer required access codes from any three of the five members of the Council in order to fire. This would've been a useful feature
 * In Stargate SG-1, the passwords of two officers are required to cancel the Self-Destruct Mechanism.
 * It varies. In 'Menace', Hammond and Carter turn it on and off with keys. In 'Lockdown', O'Neill and Kearney turn it on with keys. In 'Lockdown' and 'Avatar', Carter turns it off in the control room by tapping the computer a bit.
 * Amanda Tapping?
 * Once on Stargate a possessed O'Neill tells another guy at gunpoint to insert his key, which struck me as defeating the purpose - you'd think they'd tell the people with keys to die rather than be coerced.
 * Well, they do. Heroic Sacrifice is easier said than done, ya know.
 * Putting this trope into effect in Stargate SG-1 could almost have been considered a plot thread of its own during the first two seasons or so. In the movie and until the events of the pilot, the SGC had a hair-trigger on the self-destruct button and the gate's iris, because the primary priority was keeping everything alien out. In the show, though, they start bringing stuff back for study and all kinds of other reasons, and of course, it's stuff they don't fully understand that's often hostile. A Two-Keyed Lock is needed to stop Puppeteer Parasite, a handprint scanner is needed to stop cloaked aliens, and so on.
 * Rare example not involving nukes: in an episode of Alias set in a Romanian mental hospital, the door to get out is double-keyed, one lock on the wall to either side of a wide maintenance door, too wide for one person to turn both keys with their hands.
 * In the Doctor Who episode "Journey's End", the Earth's self destruct requires three out of five UNIT soldiers in different countries around the world to work together to activate it.
 * Referenced in an episode of Seinfeld, where George's girlfriend refuses to accept their break-up. Both she and Jerry compare this to launching missiles from a submarine (Jerry says it's not the same, but George's girlfriend says it is).
 * In the 2000s Battlestar Galactica Reimagined, the nuclear launch tubes on battlestars are controlled by two-keyed locks.
 * In the Andromeda pilot episodes, Dylan gives four people he barely knows positions on his ships - because, apparently, one person cannot launch the Nova Bombs.

Video Games

 * Metroid Prime 3: Corruption has you do this with a GF trooper in order to activate an elevator.
 * Oddly enough, only two "puzzles" in Resident Evil 4 need Leon and Ashley to do this.
 * However Resident Evil 5 has loads of these to prevent you from leaving your partner behind.
 * Resident Evil 0, considering its focus on the partner system, also had a few puzzles like this, including one literal two-keyed lock right at the end.
 * Used as a minigame in Final Fantasy VII, where to open a security door two party members must push two widely separated buttons at the same time.
 * Happens all the time in Gears of War 2.
 * In Metal Gear Solid, the player must find three card keys to deactivate Metal Gear Rex late in the game. The twist, however, is that the one key Snake obtains is actually all three keys.
 * Used in Sonic Chronicles: The Dark Brotherhood when you had puzzles where four characters had to stand in certain spots to unlock doors.
 * Unfortunately frequent in the PS2-exclusive Half Life expansion Decay, where the designers' idea of co-operative gameplay was to include puzzles that require two players to work synchronously to be solved efficiently, such as turning switches on opposing sides of a room simultaneously.
 * A few 'double-button security locks' show up in Sly Cooper 3. The timing window seems to give some margin of error. Notably, while there are many Two Keyed (and more commonly three, five, seven, or whatever-keyed) locks, these are clustered on the console as individual padlocks. Still, you do have to retrieve all the keys to open it.
 * Done, sort of, in Fur Fighters in a submarine. The player must turn two keys at the same time but instead of getting two people you just back into one key and shoot the other to make them turn.
 * In Jedi Knight: Jedi Outcast, the prison where Jan is kept is like this, with about five or six consecutive doors. Holding down the switch in between two doors holds those two doors open only, so you have to open the doors, wait for Jan to walk through to open the doors for you, and continue until you leapfrog your way out.
 * Happens all the time in Lego Star Wars. Many doors can't be passed unless two (or more) characters simultaneously throw switches.
 * Thankfully, unlike many games, the AI is smart enough to move some distance to the other switch, without having to carefully babysit them through each step.
 * Opening the shuttle bay door in Space Quest 6 requires two people (or one person ). Interestingly enough, this trope is inverted any and all of the auto-destruct sequences in the series.
 * One update of a Let's Play of the Dwarf Fortress map BoatMurdered had someone submit a drawing of the employment of the "lava death system." With two keys, natch. In the actual game, of course, the system was activated by a simple lever.
 * One bank that Sam Fisher has to infiltrate in Chaos Theory has a Two-Keyed Lock protecting its main vault. Fisher, however, has a remote-controlled key-turning device.
 * The Legend of Zelda Four Swords-series has a lot of puzzles which include the "four buttons, each of them only triggers if a person is standing on it"-factor. In some of the games this is subverted by "one player controls four characters". At some places this is required to move on, at others it's just "drop a lot of loot"!
 * In fact, there are a lot of these in the Zelda games. Sometimes using a block or the assistance of lovely Princess Zelda (as a Phantom) to push one, or hitting a series of switches with the boomerang. Majora's Mask has plenty of these in the Stone Tower, and in order to work them you need the Elegy of Emptiness, which creates a statue duplicate of your form, and the Zora and Goron masks.
 * Oddworld - Abe's Exoddus has a lot of these. Abe can turn one wheel, but there's often more than one required to unlock a door/move a platform/whatever, so you need to bring other Mudokons along and order them to turn the wheels. The timing's fairly forgiving, so assembling the Mudokons is usually the main problem... with one exception, where it's two levers instead of wheels.
 * This is one way Banjo-Tooie enforces use of the Split Up mechanic.
 * Skies of Arcadia has an odd one. A whole dungeon focused around locks that require two people to stand on panels in different parts of the dungeon. The thing is, neither side knew that they were helping each other get past and they just both happened to be searching for the same treasure at the same time.
 * Ratchet and Clank Future: A Crack in Time includes a variation of this. Clank must use his newly-acquired  to record multiple copies of himself completing various tasks (such as pressing buttons or activating platforms), usually with the end goal of opening a door at the end of a room. The actions of the copies must be perfectly timed in order for the player controlling the "real" Clank to solve the puzzle; with up to four copies working at once to complete the task, the difficulty can ramp up pretty quickly.
 * Mortal Kombat: Shaolin Monks has a bunch of these. They're completely optional secrets/goodies, and in no way necessary to continue the game. They ARE, however, essential to 100% completion. If you're a completionist, and you don't have a sibling or buddy to play the game with, don't get the game.
 * The Portal games use a few of these in both single-player and co-op. The former involve two buttons that need to be pressed within a short timespan of each other, and thus require having your two portal ends right next to them. (One of these is justified as an actual two-keyed security lock. The rest are just part of the tests.) The latter actually involve both players, and the game has thoughtfully included the ability to initiate a countdown that appears on the other's screen.
 * Okamiden being based around partners has a plenty of these throughout the game where Chibi and his partner must stand on pressure pads to unlock doors or make bridges appear. One Notable example has a two-buttoned lock which is also a trap forcing Chibi to play through half the dungeon himself to gain the key to free his friend.
 * City of Heroes (as well as some other MMORPGs) have missions where a given number of people must trigger some kind of switch simultaneously (or within a margin of error given lag times) in order to complete.
 * The Thieves' Guild vault in Skyrim has one of these, with the keys being owned by the most powerful members of the guild.

Web Comics

 * Used here in Goblins: Life Through Their Eyes, with a four-key system.

Web Original

 * In Freeman's Mind, Gordon accidentally launches what he believes to be a missile (it's actually a satellite delivery rocket, but he hadn't been paying attention to the security guard who told him about it) and afterward mentions that he would have expected one of these instead of just the Big Red Button he pressed. Even for a satellite delivery rocket, and even considering he was resuming an aborted launch, yeah, you'd think the procedure would be at least a little more complex.
 * In Axe Cop when they're visiting Magic World they have to insert two magic wands on the side of a gate at the same time to get in. Not that they get very far after that point.
 * Atop the Fourth Wall uses the exact self-destruct code from Star Trek III in the Cold Open for his review of... the comic adaptation of Star Trek III in order to

Western Animation

 * One of many many secure location tropes parodied in a Family Guy "found a new place to hide my porn" sequence.
 * On The Simpsons two keys were required to be turned simultaneously to drop the "perfect 300 game" balloon at the bowling alley.
 * In Chaotic, the Doors of the Deepmines, that act as the M'arillians' prison requires four keys held by the other four tribes to open.
 * The Fire Temple's inner sanctum in Avatar: The Last Airbender could only be opened by five simultaneous fireblasts. A single sufficiently skilled Firebender (such as a fully trained Avatar) could do all five at once, but in most cases it took five people.
 * In Gargoyles, when Halcyon Renard's second, fully-automated airship is sabotaged, he goes to the bridge to take it off a collision course, only to realize that two people are needed at two different consoles to perform emergency course corrections.

Real Life

 * Truth in Television, this, especially in terms of US ICBM silos. The two key slots are far enough apart that one person can't turn both at the same time. Plus you'll need the launch codes, if they're not set to 00000000. (Yes, that actually happened: Permissive Action Link. Under the Kennedy administration, somebody decided to put PALs on the US nuclear arsenal to prevent unauthorized firing. SAC objected to this practice, fearing the possibility that the launch codes would not be available in time of need. So, SAC very quietly installed these devices, intended to ensure the safety of the free world, and very quietly set the combination on every single one of them to 00000000. Very trusting people, SAC, although their trust appears to have been well-placed.)
 * The logic was that warheads mounted to missiles in either ground-based stations or in ballistic submarines are secure because of the two-man-rule interlocks, and PALs would risk a loss of readiness without significant security benefit. Actual (non-trivially-coded) PALs were (eventually) applied to small warheads - air-dropped bombs and ship/air-launched cruise missiles. Unlike ballistic missiles, these warheads can be stored or transported in a functional or semi-functional state, and thus can be used if lost or stolen. For these weapons, the two-man rule uses the PALs themselves - two officers must concur with the legitimacy of a nuclear launch order and release their portions of the PAL codes, or else the warheads cannot be armed.
 * The Soviets had two launch keys and unlock codes held by the higher-ups (i.e. on shore) for their submarines. Now the case for US subs, but not always.
 * For safety deposit boxes, one key is the bank's and one is held by the customer. This ensures that the bank cannot open your box without you, and that you (or someone with your key) can't open your box without showing ID to the bank. (Banks are adding increasingly more checks to get in. In the case of one large bank, you must enter a PIN, pass a biometric scan, and use a regular old key as well, constituting three-factor authentication.)
 * Some Soviet nuclear missile silos had three blast doors, each needing three keys, and each key given to a different person. So a total of nine people were needed to actually get access to the missiles.
 * In a much more mundane context, most of the rides at the local amusement park won't launch without both operators holding the go buttons, and they can still be locked out by ride sensors.
 * Many industrial machines have two start buttons, but they are close enough to be pressed by a single operator so long as he uses both hands. The goal here is to ensure that both of the operator's hands are on the control box and not in the machine. Depending on the machine, there might also be a foot pedal.
 * In some poor villages of Africa where they have opted for Food Bank (filled with food aid for use when the harvest is lean and topped up by local farmers when the harvest is good) three people from the community are given keys and required to open it. As Josette Sheeran says, food is gold.
 * As detailed on 60 Minutes, the Mount Athos monastery has a vault which contains the order's most valuable books, documents, and artifacts, the door to the vault requiring several different keys to open, and a rule in place that says a monk is not allowed to hold more than one key at one time. Mike Wallace even described it as an archaic version of the PALs system listed above.