Double Knockout

Two characters are so evenly matched that the fight doesn't end until they both land the knockout blow simultaneously.

Mutual Kill is the lethal version of this trope, and the blow does not have to come simultaneously either.

Sometimes the result of Fearful Symmetry. See also the Cross Counter. Compare Taking You with Me.

Anime and Manga
": We really suck.
 * Revy and Roberta in Black Lagoon.
 * The first fight between Ichigo and Zaraki Kenpachi in Bleach ends this way, with the twist that both declare the other victorious before toppling over.
 * In Yu-Gi-Oh! (both in the real card game and the anime), when monsters of equal attack points battle, they both die.
 * The card game takes it a step further with a monster called Exploder Dragon, the whole point of which is to kill whatever kills it. If the player controlling it attacks, neither player is hurt.
 * Double KOs aren't too uncommon in Pokémon either -- it's what attacks like Explosion are made for. In multiplayer matches, whoever did the attack loses. In the single-player game, the player always loses because there would be no more Pokémon to fight with in another battle. Of course, the anime uses the less cheap method and does Dragonball Z tactics to get this result (attack meets attack, and it all blows up).
 * This happens in the anime quite a lot.
 * In Naruto, Sakura and Ino KO each other during the Chunin Exams.
 * Chouji uses his family's Dangerous Forbidden Technique to knock out kill his opponent in the Sasuke Retrieval arc. The transformation accompanying the finishing move is Cast From HP.
 * In Outlaw Star, Gene and Shimi fought each other and they both got knocked down in one shot.
 * Kinnikuman has several instances of this:
 * Terryman vs. Ashuraman: When both fighters are knocked out of the ring, Terryman, realizing he can't beat Ashuraman, holds him to a draw by trapping him in a scissor hold so he can't get back to the ring before being counted out.
 * Terryman vs. 100-Ton King: After Terryman performs a Calf Brander on the 100-Ton King, his metal body shatters into pieces. It looks like Terryman is the winner...until King's fist hits him on the head, knocking him unconscious.
 * Mammothman vs. Cannonballer: They clothesline each other simultaneously. However, this only happened because Mammothman was taking a dive so Super Phoenix could have the pleasure of beating Big Body personally.
 * Ashuraman vs. Satan Cross: Ashuraman knocks Satan Cross into the arena wall with his Ashura Lightning Strike, but Satan Cross counters with a Triangle Dreamer as they fall to the floor, leaving them both KO'd.
 * Brocken Jr. vs. Prisman: Brocken Jr. knocks Prisman out of the floating ring they're fighting in with his Bremen Sunset, shattering his head against a statue.
 * Robin Mask vs. Mammothman: Robin actually defeats Mammothman with his Ropework Tower Bridge,
 * Samurai Champloo:
 * You got that right."

"Xin: I admit... that was tough..."
 * The first meeting between Masaru and Agumon in in Digimon Savers involves a variant of this, in that instead of the two just knocking eachother out their fight lasted until both collapsed from exhaustion.
 * Two of Ranma's encounters in Ranma 1/2 ended this way:
 * When battling Ryouga and his new ki attack, the Shishi Hokodan, Ranma used the downwards force of the perfected form to punch his rival in the face, just as the latter was made vulnerable to the Shishi Hokodan. Ryouga was KO'd instantly, and Ranma had only a few seconds to deliver a pithy remark before falling over.
 * When Pantyhose Taro's monstrous form gained tentacles, she lured him to the smokestack of a nearby bathhouse so the tentacles would climb into the chimney and get stuck. Unfortunately for her, Pantyhose Taro simply ripped off the smokestack and smashed her in the head with it... and the momentum carried it into his own head. Both of them were knocked out and fell in the bathhouse, regaining consciousness (and their true forms) with the hot water.
 * Goku and Baby in Dragon Ball GT, and they stay unconscious for a whole episode.
 * A major battle in the Magic World arc of Mahou Sensei Negima ended in this manner, as managed to take everything he threw and still managed to stand up for  that ended with both sides unconscious.
 * In Magical Girl Lyrical Nanoha ViVid, the battle between turned out to be like this. After the latter walked away in an apparently easy victory and transformed back to her normal self, the full extent of her injuries came crashing down on her, causing her to black out instantly..
 * In Xin, the eponymous character and Dom manage to knock each other out, though Xin does his knock out in a far more spectactular way.


 * In Fairy Tail, after
 * The final battle between Kallen's Guren SEITEN and Suzaku's Lancelot Albion in Code Geass ends with Kallen ramming the Guren's left arm through the Lancelot's chest just as the Lancelot's slash harkens tear off the Guren's head and right arm. Afterwards, the Guren falls off the platform they were fighting on, just before the Lancelot detonates. and supplemental materials show the Guren being rebuilt.
 * The fight between Lockon and Ali ends this way in the first season of Mobile Suit Gundam 00: when Ali figures out that Lockon doesn't have periphery vision on the right due to an earlier eye injury, he uses that to knock Lockon's Gundam out of the fight with a surprise attack from the blind angle. However, Lockon abandons the Dynames and commandeers the particle cannon of the destroyed-earlier GN Arms which he uses to blow Ali's mobile suit out of the sky but not before Ali's parting shot hits the cannon.

Comics

 * This is how Superman's battle with Doomsday in The Death of Superman ended.
 * The fight between Cleverdix and Majestix in Asterix and the Great Divide.

Film

 * Just barely averted in Rocky II, when Rocky gets to his feet at 9 and Apollo can't make it. A very rare but occasional Truth in Television for real life boxing and MMA type matches as well.
 * The same is nearly true in the Blur short film A Gentlemans Duel. A Frenchman and a Britannian face off in steam-powered death mechs, each new feat (grapple, toss into building, fist-onna-chain, electric fish coil, poodle cannon) is countered by another attack until finally both mechs are pushed to the limits of their overdrive, shred each others limbs, crash together to knock the other over, and explode. Both gentlemen crawl from the wreckage and inch across the ground until they bump heads and vaguely try to start fighting again.
 * Kick-Ass and in Kick Ass. However, Kick-Ass manages to get up soon after.

Literature

 * Slaves of the Mastery has a ritual fighting form in which one of the most spectacular maneuvers is to not block the opponent's strike (both participants wear cat-claw style weapons), but to anticipate the impaling, charge, and impale the opponent while simultaneously being impaled.
 * Not strictly a knockout, but in the children's book Mr. Biff the Boxer, Mr. Biff and Mr. Bop suddenly "[biff] and [bop] each other out" in the first round for a simultaneous ten-count.
 * Subverted in Matthew Reilly's Scarecrow. Schofield and Wexley hit each other with withering blows almost simultaniously, fall to the floor... and keep fighting!
 * Gawain and Ywain do this in one medieval romance; Gawain actually does things like it in lots of places because he was the baseline of awesome, and putting your guy up against Gawain without running into Mary Sue syndrome was a generally effective move. (Lancelot wins, by the time he gets his. In this case, they fight until they fall off their horses, and then fight until their armor and weapons (though not helmets) are wrecked, and then fight until they can't get up anymore. Then one bloody mess asks the other bloody mess his name, and it turns out he's nearly killed his best friend. Crying and kissing ensues. Seriously.
 * Note that apart from the kissing, this could have happened in a modern fighting anime and you wouldn't know the difference, but for the French poetry.
 * Much later, in Malory's Le Morte Darthur, some while before Gawain and Lancelot have their climactic battle, Gawain and Arthur hear that Lancelot's killed Gawain's younger brothers who were actually on his side without noticing, and do this...synchronized swooning thing. Twice.

Live Action Television

 * A Double Knockout occurred between two boxers in Sgt Bilko due to the fact both boxers were under in impression they were suppose to take a dive.
 * Taken to the extreme, since only one of the boxers took a punch "out of boredom". Immediately after it connected, both boxers (the guy who got hit and the guy who did the hitting) took the dive.
 * A similar scenario happened in a prison boxing match in an episode of Porridge. In that instance, they had both been bribed.
 * Happened at the end of the three-way phony brawl between Conan O'Brien, Stephen Colbert, and Jon Stewart back during the writer's strike.
 * Happens to Francis and Lavernia in one episode of Malcolm in the Middle.

Professional Wrestling

 * Not uncommon in Professional Wrestling as well, especially if a title or plot point is on the line.
 * In the Triple H/Shawn Michaels Last Man Standing match at Royal Rumble 2004, both men were knocked out and neither was able to make it back to his feet. This happens in about half the LMS matches.
 * Happens a little in battle royals as well, including the Royal Rumble itself. There was that one infamous Royal Rumble where Bret Hart and Lex Luger took each other over the ropes at the same time. Cena and Batista eliminated each other in the '05 Rumble faux finish.
 * Professional Wrestling can also have the Double Pin, or the even rarer double Pin/Submission, as with Kurt Angle being pinned while making The Undertaker tap out to a triangle choke.

Video Games

 * Double KOs are possible in every Fighting Game, make no mistake. Mostly because, since it's technically possible in any fighting game, the designers have to program a way to deal with it if it happens. However, some games treat them differently. While the Street Fighter and King of Fighters (and Samurai Shodown) series would count that as a win for both fighters (and would probably go into sudden death if it happened both times), the World Heroes series doesn't count it at all.
 * In Street Fighter, when there are more than two double KOs in a row and no clear winner by the fourth round, both players lose.
 * Some lower-quality Fighting Games arbitrarily declare the CPU the winner in a Double KO.
 * This can also happen in games where it is possible to defeat your opponent by knocking them off the platform you are fighting on. In Soul Calibur, a player who falls off in the course of knocking someone else off in the process typically ends in a Draw.
 * However, this only works if both characters fall at the same time. If one person falls off the edge just a few seconds after the first (with rather hilarious animations for the event), then the second person to fall is declared the winner.
 * Speaking of Soul Calibur, Yoshimitsu's seppuku move seems to be tailor-made for these.
 * Soul Calibur IV has an achievement/trophy for getting a Double KO: Two Cannot Exist Together.
 * Blaz Blue deals with this by giving each player a match point, unless said match point makes one player win the entire set. In that case, only the player who's behind gets a match point. If both players need one match point to win, the game continues into overtime instead of ending in a draw.
 * Even Super Smash Bros Brawl has Double KO potential in Ganondorf's "Flame Choke" move, which can result in sudden-death even after a stock battle.
 * This is, in fact, a viable strategy: Bowser, Ganondorf, Kirby (less viable after the nerfs), Metaknight (harder to do than others), King Dedede, and a few others can easily win a match by getting the first kill then proceeding to suicide their opponents. Strangely, Kirby and King Dedede are capable of avoiding their own demises with their Inhale ability. Bowser is amazing at it as his animation for this puts him at a higher altitude than the opponent, meaning they often die first (which subverts this trope only to invoke another.)
 * This was Kirby's least loved tactic during the original Smash Bros. Grab them near a ledge, and then hit up, forward, or backwards (as appropriate) and fall to your demise (and theirs).
 * One game that didn't have any kind of Double KO handling was WWF Raw for the SNES. Because there were only two ways to win, pinfall and count out, there was no way to have a double KO in a 1v1 match. However, if you were playing a tag match where one teammate got a 3 count pin just as his partner got counted out of the ring (thus disqualifying his team), the game would dim to a dark gray and hard lock until you reset.
 * Can happen in some other games, also--but of course, going down while taking the boss down counts as a loss. One famous example of this is the Egg Viper in Sonic Adventure, which specifically tries to take the player down with it. It's also possible to be KO'ed by the device that shoots the spiked ball falling off of the Zoness boss at the end of the battle in Star Fox 64, forcing a replay of the entire second half of the level.
 * World of Warcraft, due to its asymmetric client-server model, has this sort of thing happen all the time, due to the order of events being processed differently by the server and each respective client. A classic example is two characters stunning each other simultaneously with instant attacks. A simpler case occurs when both opponents are capable of applying damage-over-time effects; this is an iconic feature (to the point of Memetic Mutation) of battles with Warlocks.
 * Fighting a Warlock generally means that you end up killing them, going home, taking a shower, making a sandwich, and dying while eating it.
 * The particularly thorough fan-made Kinnikuman game Muscle Fight has a number of moves that will drain your health, possibly all the way down to nil. However, provided the opponent is beaten with said move, it counts as a win. Thankfully, said moves are treated as last resorts that require *very* little health, sometimes as little as 1/16th, and victory is usually assured if one of these connects.
 * Double (more like Mass-mutual) KOs are all too common in Bomberman multiplayer, due to the hectic pace of battle.
 * Next-to-last boss in Purple literally mirrors your characters movement, except that his frisbee dissappears earlier. The strategy for easy defeat.
 * Due to a change in the combo and hit detection system in Streets of Rage 3, it's possible for you to take out an enemy's remaining life and for the enemy to do the same to you at the same time. This can be quite a funny moment if you end a boss battle this way.
 * Some enemies in RPGs have a Last Ditch Move they will perform upon death. Depending on the move, it is possible for the enemy (or its party) to use a move that will wipe out your party instantly as they die, resulting both sides in being knocked out. However, this counts as a Game Over for you because you didn't survive.
 * Notably, one boss (Magi Master) in Final Fantasy VI would use Ultima just before dying. It was powerful enough to deal damage near the cap, and it hit everyone in your party. The only way to reliably win the fight is to case Reraise (Life3) on at least one of your party members, which revives a character automatically on death. And considering Reraise is only gained from one Esper, it required some grinding to get it.
 * If you don't have Reraise or the Esper that grants it, the only other likely option is to use Rasp/Osmose to drain Magi Master's MP, making him unable to cast Ultima (also, like a few other magic-based enemies, he dies when he hits 0 MP). The fight takes an extremely long time, but seeing "Ultima...Not Enough MP" flash on the screen, followed by the boss's death, is entirely worth it.
 * Having a character use the Jump command also helps since the character jumps off screen and avoids all attacks.
 * As mentioned in the anime section, double KOs aren't too uncommon in Pokémon either -- it's what attacks like Explosion are made for. In multiplayer matches, whoever did the attack loses. In the single-player game, the player always loses (regardless of who initiated the Desperation Attack ) because there would be no more Pokémon to fight with in another battle.

Web Comics

 * When Aylee and Bun-Bun first meet in Sluggy Freelance, they quickly get into an all out brawl that lasts for hours on end and involves firing bazookas and ripping off arms. Eventually they both collapse at the same moment, exhausted.

Western Animation
"Professor Farnsworth: Now, now. Perfectly symmetrical violence never solved anything."
 * Leela-1 and Leela-A performing a flying kick at each other in the Futurama episode "The Farnsworth Parabox".

"Announcer: (after a double knockout) Phineas WINS! Phineas: I'm kicking my own butt!"
 * A Non Video Game example appears as a joke in Phineas and Ferb. Sort of. An episode has the Phineas, while sick in bed, playing a fighting game with his other invalid friends. Since everyone else fought against each other he has to go against himself. The result?

Miscellaneous

 * Possible in the card game Magic: The Gathering (among many, many others) -- both players can hit zero or less life and it's a draw. Later editions of the game partially avert this (a "loss" counts at the end of an action rather than the end of a phase, making the window for a draw that much narrower).
 * Many scenarios allow it in the Yu-Gi-Oh! CCG, though, and is in fact the purpose of a tournament-specific strategy using Self-Destruct Button (a card that reduces both players life points to 0 if your opponent's life points are a good deal higher than your own); as long as at least one duel in a match is a win, all other duels in that match can result in a draw and still count as winning that match.
 * While not technically a knockout, the epee match between Phule and the Red Eagles commander (whose units were tied 1-1 in this particular competition, and 1-1 in the overall competition, making this duel the absolute tiebreaker) in Phule's Company ended with both opponents striking each other simultaneously on a sudden death point, meaning they have both scored the winning point. Under tournament regulations, this is rules as a double loss, which means that Omega Company and the Red Eagles tied, 1-1-1 each.
 * In the board game Stratego, equal-ranked pieces capture each other in duels.
 * A real life example in Mixed Martial Arts.