Mutual Disadvantage

"So what now, Jack Sparrow? Are we to be two immortals locked in an epic battle until Judgment Day and trumpets sound?"

—Captain Barbossa, Pirates of the Caribbean: The Curse of the Black Pearl

A Mutual Disadvantage is a scenario in which two (or more) sides in a fight are unable to gain an advantage over each other due to both being equally disadvantaged against one another in either offensive or defensive abilities. This can come in two flavors;

Mutual Invulnerability: Both participants are offensively disadvantaged - neither of them can harm the other, as they are both immune or resistant to each others' attacks. The result is a battle between two Stone Walls, making for a stalemate unless and until someone else intervenes.
Mutual Weakness: Both participants are extremely susceptible to each others' skills, essentially making it a fight between Glass Cannons. The result is either a standoff wherein both sides try to avoid Mutually Assured Destruction, or a fight that could go either way, but is very likely to be one-sided in favor of whoever gets the first strike. Compare Mexican Standoff, where the stalemate is the result of the participants all being perfectly matched. Often happens in fights against Evil Counterparts.

Note that any RPG system with pair-based Elemental Rock-Paper-Scissors will naturally display both types, with equal elements creating mutual invulnerability while opposing elements have mutual weakness.

Like Cannot Cut Like and Takes One to Kill One are Sub Tropes.

Examples of Mutual Disadvantage include:

Mutual invulnerability

Anime and Manga

In One Piece, Smoker and Ace proved unable to hurt each other, due to both being logia fruit users with elements that do not naturally interact - all they achieved was a huge pillar of smoke and fire.

Comics

The most recent issue of Billy Batson and the Power of Shazam; Captain Marvel and Black Adam are evenly matched and cannot harm each other; likewise Mary Marvel and Black Adam Junior who is an embittered Freddy Freeman! seem to be immune to each other's punches. But Black Adam Junior can injure Captain Marvel.
Havok and Cyclops in X-Men comics. As they are brothers, this situation happens very rarely, but when it does, they are both immune to each other's attacks, because their powers work in the same way, and each can use the other's attack as an energy source. The same is true of Sean Cassidy (Banshee), and his cousin Thomas, neither's powers can affect the other. This is believed in both cases to be due to their genetic relationship and can apply to other related mutants as well.
In a Marvel/DC crossover between Batman and Captain America (comics), after The Joker becomes furious after finding out that his collaborator the Red Skull is a Nazi, the two villains try to use their own weaponized toxins on each other to no effect. Since both Joker and Red Skull are immune to their own toxins and both Joker Venom and Red Skull's "dust of death" are remarkably similar, both characters are also immune to each other's toxins.

Film

Barbossa and Jack Sparrow fall into a type one version of this trope for a short while, during Pirates of the Caribbean: Curse of the Black Pearl.
- Again in the third film, though here imposed by terrain. Calypso hated both ships and their captains, so she created a huge typhoon that made maneuvering impossible (removing the Black Pearl's speed advantage) and made firing the cannons impossible (removing the Flying Dutchman's firepower advantage). Thus it came down to a brutal ship to ship fight between crews she hoped neither would survive.
In the final The Matrix movie Neo has fully embraced his 'The One' status and therefore can do just about anything in the Matrix and can withstand an almost limitless amount of damage. Agent Smith has infected everyone, human or machine, in the Matrix and can also do anything and withstand anything. Made for a really boring fight sequence.
- In no small part because most of the viewers were expecting a city-destroying, 1 vs Millions brawl to the end. And instead got two guys flying in circles.
The Golden Voyage of Sinbad. The eternal battle between Good and Evil is personified by a combat between a griffin and a centaur at the Fountain of Destiny. Neither one can prevail on its own, but "The deeds of weak and mortal men can tip the scales one way or the other."

Literature

Megamorphs #2: The crab aliens can't get to the ant aliens' base without getting shredded by their energy weapons, the ants can't penetrate the crabs' force field.

Live Action Television

In one episode of Charmed Piper and Cole have a fight while both are invulnerable.
In an episode of the Superman TV-series, some random dude off the street ends up with a carbon-copy of Superman's powers due to lightning. Said random dude then starts a Hero For Hire business, charging people premium rates for applying his superhuman powers, in between using his X-ray vision to check out girl's changing-rooms. Supes gets fed up with this pretty quickly and starts actively interfering in the 'business' (and pleasure), causing the dude to lose his temper and start a fight. Both of them, of course, are super-strong, but entirely invulnerable - and they're more invulnerable than super-strong, resulting in a Type-1 brawl... the only possible loser being the surrounding area.
Caprica at one point had Zoe and Tamara fighting in V-World. In this case, they can hurt each other all they want, but neither of them can die since they're both digital avatars. The fight only ended when Zoe talked Tamara into an alliance.
Battles between Ascended beings usually work out this way in the Stargate Verse. Since they're Energy Beings of equal stature, neither one can do anything to overpower the other, but they can keep the other sufficiently distracted as to be unable to do anything else. Both Anubis and Adria are taken care of by getting locked into an eternal fight with another Ascended.
- It is implied that Ascended beings can kill one another but only if one is vastly more powerful than the other, which is a very rare case but not impossible: belief can empower an Ascended which is why they insist to everyone on the mortal plains that they're not gods. That was one of the causes for the ideological split between the Alterans and the Ori: the latter formed Origin specifically to gain even more power but the distance between the Ori galaxy and the Milky Way meant they were still too weak to overpower the Alterans... which is why they started converting the Milky Way's native population to Origin as well, abusing the Alterans' Prime Directive to build up a power base in plain sight without a Deus Ex Machina to fear from.

Professional Wrestling

In ECW Rob Van Dam and Jerry Lynn usually started their matches with a series of Chain Wrestling moves which the other countered pretty quickly.

Real Life

England vs. France in the Hundred Years War, among others. England had the world's best navy; France had the best land army. England was unable to launch a successful land war against France, while France was unable to even touch English shores.
A mention of the Peloponnesian War could probably go along with that France/England Hundred Years War example. Athens was a sea power and Sparta had the best land forces in Greece. Sparta had to build a navy before it could successfully choke off Athens, almost 30 years in.

Video Games

In Team Fortress 2 this is the case with a few of the classes.
- Two Pyros will fall into this trope. Both are resistant to each other's signature ability (setting each other on fire), their unlockables are useless as the Flare Gun and Axtinguisher require their opponent to be on fire , meaning the winner is decided by either shotguns (if either has one equipped), flamethrowers (which would require either participant to move into the range of the other's flamethrower) or melee weapons (which do have some advantages over each other but don't make all that much difference).
- Demomen equipped with the Chargin' Targe might also qualify (since it increases their resistance to explosives and removes their ability to ambush enemies with sticky grenades).
In Pokémon, many elemental types (fire, electric, ice, and so on) are resistant to attacks of their own element (receiving half normal damage). Bug and Fighting types are resistant to each other's element, and Normal and Ghost types are completely invulnerable to the other's element. A fight between such types, especially at low levels (where they're unlikely to know any other attacks), will often result in this.
- Wobbuffet is another example, as it learns no direct attacks whatsoever and is capable only of various Counterattacks. Pitting one Wobbuffet against another Wobbuffet will result in a stalemate where neither one can harm the other in any way (and in Generation 3, neither could be switched out for another fighter due to the opponent's ability). However, once they both deplete their PP (which is going to be a while) they will resort to Struggle as their attack of choice... except, if they both hold Leftovers (and they should), their damage will be outhealed. TL;DR: fight never ends. And this is actually why Wobbuffet is banned.
- And we musn't forget Ditto. His only move is the ability to transform into the opponent, copying their appearance, stats (barring HP), and their moveset. Now, let's pit two Dittos against each other...
In Escape from Monkey Island, you engage in a giant statue fight with the villain. Your life bars regenerate too fast for either side to win, so you have to figure out an alternate means to proceed.
In Touhou, mortal enemies Mokou and Kaguya, while not invulnerable as such to each other's attacks, are both immortal and will regenerate the wounds inflicted by the other. The result is a long-term stalemate where no matter who wins the individual battles (usually Kaguya), either of them will be back to start again in no time.
Knalgan mirrors in Battle for Wesnoth - a mirror match with a faction composed of highly defensive and slow dwarves, and fast but low-damage or frail outlaws. Neither side has and magic or poison needed to dislodge the other's defensive dwarves, and neither side has any Time-of-Day based powerphase advantage over the other. As a result, neither side has much incentive to attack.
In the vast majority of MMORPGs there's the quintessential healer vs healer "battles". Extended in some cases when defensive non-healing classes and setups have some mean of regeneration and extreme defense (as opposed to healer's low defense and extreme regeneration).

Web Comics

The titular characters of the webcomic Bob and George were immune to each others' powers due to being brothers. This is in turn a reference to Scott and Alex Summers being immune to the other's superpowers for the same reason.

Mutual weakness

Film

In Star Trek II: The Wrath of Khan, the still badly-damaged Enterprise leads the less-damaged Reliant into the Mutara Nebula. When Saavik points out that the nebula will disable shields and sensors on any ship that enters, Spock answers "Sauce for the goose, Lieutenant. The odds will be even."

Video Games

Also in the Pokémon series, Ghost and Dragon typed Pokémon are notably vulnerable to their own element, especially the Dragons as they are the only element that Dragon-type attacks get an advantage against at all. It is also common competitive practice to teach a given Pokémon moves from other elements specifically to counter the types they are weak against (such as giving a Fire-type user the Grass-type "Solarbeam" for use against Water-types).
- Due to the complicated nature of Elemental Rock-Paper-Scissors, Pokémon with dual elemental types can in fact become vulnerable to one of their own elements (like the Rock+Flying Aerodactyl being vulnerable to Rock attacks, or the Fighting+Steel Lucario being vulnerable to Fighting attacks).
- When you start dealing with dual-typed Pokemon, there are a lot of possible matchups between different Pokemon that are weak to one of their opponent's types and strong against another (like the Dark/Fire Houndoom versus the Water/Ghost Jellicent - Water beats Fire and Dark beats Ghost).
- Pokémon Diamond and Pearl's use of dual-typed starter Pokémon provides an interesting scenario, where each member of the starting trio ultimately acquired a secondary type that could counter the opposing element: Torterra's Ground typing can counter Infernape's Fire, Infernape's Fighting type can counter Empoleon's Steel, and Empoleon's Steel at least removes its normal weakness to Grass. Infernape was still the fastest of the bunch, though, sometimes capable of knocking the opponent out before they could even throw their first attack.
- Gastly's family is interesting in particular in that the only targets he deals super-effective damage with ghost attaks to - that is, Psychic and Ghost - do super effective damage to him as well. Poison attacks on the other hand - oh, wait, nevermind, he does not learn any without TMs.
The Undead and Drake mirrors in Battle for Wesnoth - most Undead units are vulnerable to the same faction's magical Dark Adept unit and its Arcane damage, and similarly all Drake units are vulnerable to that faction's magical Cold damage dealing Saurian Augur. Both the Adept and the Augur, however, are Squishy Wizard units vulnerable to the conventional attacks of their respective faction. The result is Mutual Weakness.
Other Class versus Class battles in Team Fortress 2 are more of a Type 2, coming down to who has the better aim, ping, and/or luck (Sniper wars in particular).
In many Final Fantasy games, the air/wind element is often vulnerable to itself, inflicting greater damage on flying enemies.
- Also, Fire and Ice elements are usually vulnerable to each other.
In Final Fantasy Tactics characters could receive attack (or defense) bonuses depending on each fighter's astrological sign—however, the bonus always applied both ways.
In the Dissidia Final Fantasy campaign modes, the player would sometime happen upon a golden Manikin piece. These units tend to be equipped in such a way that their health is critically low, but are augmented in other ways (such as having nigh impenetrable Bravery defence, or having summons that send their own Bravery points skyrocket), meaning that the first HP attack to connect is generally what wins the match.

Real Life

The concept of idea of mutual assured destruction is the reason the Cold War "remained cold"; if either the U.S. or Russia launched a nuclear attack, the other side would have enough time to launch a nuclear counterattack and hit the other side just as hard.
- This is still generally the reason nobody uses nuclear weapons in the modern age: even if you did launch a nuke at somebody else, they'd just lob a nuke right back before yours got there, and then you'd both be dead.
- This also served as the central conflict in the film WarGames when the WOPR supercomputer (with access to real nukes) was instructed to play a simulation called "Global Thermonuclear War". In the end, it memorably declared "The only winning move is not to play."
Any duel with lethal weapons where the participants are not wearing armor can become this, especially if the weapon style is focused on offense. For example, a LOT of serious rapier duels ended with either two people dead, or one dead and one crippled.

Pair-based systems

Video Games

Chrono Cross had three sets of opposing elements (Red/Blue, Green/Yellow, Black/White), each dealing increased damage to its opposite and reduced damage to itself; every character in the (rather large) roster was assigned an elemental affinity.
Final Fantasy X and its sequel, although the player characters were Non-Elemental by default, elements were arranged in pairs (Fire/Ice and Water/Thunder) with each doing increased damage to its opposite. The player could also find (or customize) weapons and armor with elemental affinities, which could create scenarios of mutual weakness or invulnerability depending on what was equipped at a given time.
The Legend of Dragoon had pairs of opposing elements (Fire/Water, Earth/Wind, Light/Dark) doing increased damage to each other, reduced damage to itself; the Thunder element had no opposite, but also shared reduced damage against itself. Every character (and most monsters) had an elemental affinity, and each character's strongest armor (the "DG armor" series) would completely nullify damage from their own element.