Truth in Television with Nuclear Weapons. The places where each key is used are always placed too far apart for one person to activate both or all of them on his or her own. 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. May sometimes be played for laughs, where the thing being protected by the two keyed lock turns out to be something mundane, or even silly. Not to be confused with Double Unlock, which is about enabling new game features.
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- 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.
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!!!
- 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. This was reduced to 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 Striker S 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.
- The lab where the extremis samples were kept in Iron Man: Extremis. This was a key plot point.
- Terminator 2: Judgment Day had a two-keyed lock 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, even though it was just a drill. 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: Dark Territory, 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 honey production in Bee Movie.
- In Dr. Strangelove, the pilot, bombadier and electronic warfare officer each have to have to operate the first and second safety switches to arm the bombs.
- 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 Golden Eye had this for the titular weapon as well. Interestingly, this was replicated in the secret underground base as well.
- Well, you don't want Boris setting it off early.
- 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, to the point where Chekov has to declare himself "acting Science Officer" rather than his normal position as chief of Security.
- 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.
- The safe deposit vault that Artemis Fowl burgles in The Opal Deception has a lock system like this. Artemis gets around it using an ingenious extending pole disguised as a collapsible scooter (to get it past security).
- An escape hatch in Thursday Next: First Among Sequels has two handles which need to be turned simultaneously.In a touching Redemption Equals Death, Evil Thursday chooses to help Thursday escape, knowing that she herself has no way out.
- 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 Pyx is locked with three different locks and, later, kept behind two doors which are each locked with three locks. That doesn't stop Saturn 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 computer terminal just as another one breaks through the software defenses.
- From the Coldfire Trilogy, the vault in which the holy relics of the failed Crusade against the Forest are stored behind a two keyed lock. The Patriarch holds one key.
- In The Good Thief's Guide to Amsterdam, each of three thieves keeps one of the three keys needed to open the safety deposit box that hold the fortune in diamonds that they stole. One of the three thieves hires the protagonist Charlie to steal his partners' keys.
- One can actually see this decay in Star Trek. In the days of the original series, voice-print match and codes from three senior officers are necessary to activate the auto-destruct sequence. In the Next-Gen days, it only only takes two with biometric hand scans, and requires both to disable it. By the time of Voyager, Janeway could apparently destroy the entire ship on a whim.
- This actually becomes a plot point in "11001001". The Bynars load the Enterprise's main computer with all the data from their home planet. In order to download it back into the planet's mainframe, a passcode needs to be entered. However, since the Bynars always operate in pairs the passcode has to be entered at two separate terminals simultaneously.
- 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 when three (and eventually four) of the Xindi races back out of the plan to destroy Earth. Unfortunately, the reptilians had ways around the codes.
- 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.
- 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. Sydney deals with this by acrobatically turning the other one with her foot.
- 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.
- Averted in "The Day of the Doctor", when Kate Stewart is able to order the destruction-by-nuke of the Black Archive by herself.
- 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, 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 even though one can arbitrarily give out the positions necessary to do so.
- Pops up again in the last season where a pair of (stupid) gang leaders join forces to shake down the locals. They find a key to open a ship they want. Only at the moment when they both have to enter the keys to get the door open they get greedy and turn on each other.
- During the first season finale of seaQuestDSV, Captain Bridger and Commander Ford use their keys to arm the ship's nuclear warheads as a Self-Destruct Mechanism to seal a massive crack in the ocean floor.
- In the second V miniseries, the Visitors intend to deploy their mothership as a nuclear bomb as a last resort in case their troops can't hold on to Earth. John and Diana both take one of the arming device's two keys. John is willing to concede defeat and just retreat, but a spiteful Diana wants to kill off all of humanity because they defeated her. She orders John to hand over his key at gunpoint, and kills him when he voices his disgust.
- Metroid Prime 3: Corruption has you do this with a GF trooper in order to activate an elevator. The "trooper" in question is the shape-shifting foe Gandrayda.
- 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.
- Resident Evil 6 has random doors all over the place that require two people to hold down buttons on either side of the frame for them to open. Nothing valuable is behind them, they're just another way to make sure you don't leave your partner behind.
- Used as a minigame in Final Fantasy VII, where to open a security door three party members must push three widely separated buttons at the same timenote .
- The Four Shrines in Final Fantasy IX must be activated simultaneously, but the only impact this has on gameplay is forcing you to fight a boss with just two characters; you don't control the rest.
- Done in Modern Warfare 3 in a mission where you play as Frost and have to infiltrate a Russian submarine. The level has you launching missiles from the sub onto the other Russian boats, but before you can do so Frost and Sandman have to use a two-keyed lock to open the button that launches the missiles. Sandman also apparently gets both keys from the Captain, which does seem to defeat the point; but then again, they aren't nuclear missiles.
- 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. The key is made of shape memory alloy, so depending on what extreme temperatures Snake subjects the key to, it changes into each of the keys.
- There's also a two-part code, each part memorized by DARPA chief Donald Anderson and ArmsTech president Kenneth Baker, respectively. The PAL key option is stated to be a backup, as a way to disable Rex once the code has been entered, or to enable it if the code is somehow lost. Snake is led to believe Psycho Mantis extracted Anderson's code, even though it should have been impossible, while Baker gave in to Revolver Ocelot's torture. It turns out Ocelot inadvertently killed Anderson before getting him to break, requiring Decoy Octopus to impersonate him and have Snake figure out the key system so they could activate Rex.
- Snatcher has a three-keyed one in located in Queens Hospital.
- Used in Sonic Chronicles: The Dark Brotherhood when you had puzzles where four characters had to stand in certain spots to unlock doors.
- This is also a common type of puzzle in Sonic Heroes, where all three teammates have to stand on switches at the same time to activate a door or gimmick. While at the start these puzzles boil down to running through a line of switches in Power formation, later stages have the switches located in completely different corners of an area, resulting in a hybrid Escort Mission and No-Gear Level where you have to leave your teammates behind at switches as you travel to the next while making sure said teammates remain safe.
- 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 3: Honor Among Thieves. 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 II: 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 adaptation games. 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 with another person's arm appendage). 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.
- One way Banjo-Tooie enforces use of the Split Up mechanic is by having doors that open only when two Pressure Plates are occupied by a bear and bird at the same time.
- Skies of Arcadia has 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 & Clank Future: A Crack in Time includes a variation of this. Clank must use his newly-acquired ability to shift time 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.
- Ōkamiden 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. No one realizes it's already been emptied by Guildmaster Mercer Frey, using a magical lockpick he stole from the goddess Nocturnal.
- In the "purple world" of Braid a few parts require synchronized or simultaneous flipping of switches or unlocking of doors, using that world's gimmick.
- The Dungeoneering skill in RuneScape has two variants on the Two-Keyed Lock, in the lever room and follow-the-statue-leaders emote rooms.
- An earlier example would be getting to the king dagganoths, which requires two people to get past any of the three entrances to the monsters, then three people (each having gone down a different path) to open the door to the kings' lair.
- Present early on in Deep Fear, as you have to get the two keys from the people charged with their safekeeping (both incapacitated) to abort a nuclear missile launch from a damaged submarine. Although, both slots are within arm's reach from a single spot...
- Fatal Frame II / Project Zero 2 has a couple of places where the twins have to stand on pressure pads to unlock certain doors. Seeing as the village had been completely cut off from modern technology it's rather out of place to the general rural-rustic style.
- Darksiders II has few. Several dungeons has Death commanding up to ghosts which results in many cases of this. Then later in the game Death gains the ability to split his soul into two which generally follows this trope more clearly as you have to control both to perform the same actions at the same time.
- It also turns out the that the final door in the game requires two keys, and they have to be turned at the same time for the door to open.
- The Cave has several puzzles require characters to pull switches/stand on pressure pads to allow other characters to past. Also a literal three-keyed-lock is in the scientist level.
- A small example in Mega Man X: Command Mission: the intro stage involves waiting for a certain tone from Zero to hit a button and unlock a set of doors. Subverted in that Zero is controlled by the game and not the player. And your reward for messing up the timing? A battle against three Preons.
- A very common puzzle type in Deadly Rooms of Death. As you only control one character, such puzzles generally require manipulating monsters so they move onto pressure plates at the correct time.
- Both castles of Dhaos in Tales of Phantasia feature doors that can only be opened when 4 (and later 5) people stand in front of them, basically ensuring that the party doesn't stay separated for long.
- Hotel Dusk: Room 215 has you fix the lights to a room, which requires that you turn on both switches required to allow the lights to turn on at the same time. This would be fine if it weren't that the touch screen works against you.
- In Spider-Man: Shattered Dimensions, the 2099-verse frequently requires two or more scientists working at separate consoles to open a door. Your job is to keep them alive until they've done it.
- Syndicate (2012): The door to the Eurocorp Data Core is protected by one of these, theoretically making it the most secure room in the entire facility. However, because both key terminals are not immune to hacking and the timespan needed to activate both locks is rather generous, a single Agent with a DART 6 chip can hack both locks rapidly in succession, completely bypassing the security system.
- One quest in The Elder Scrolls IV: Oblivion involved a trio of treasure hunters protecting their loot with, among other things, a chest with three locks, with one key per hunter. Predictably, one of them gets greedy and murders the other two for their keys. After hunting him down and killing him, you get all three keys from his corpse and, thus, their loot as your reward.
- 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 prevent Lord Vyce from taking his ship back. Vyce manages to stop the sequence before the ship blows up.
- Possibly unique in that one of the people who confirms the code is the ship's AI. Linkara makes it clear that he's not asking her to help kill herself, though, as she can transfer her consciousness off the ship.
- 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.
- 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.
- Joked about in a "What If" on xkcd with the National Weather Service having a special snow measuring board;
It's snowing. We'd better go get the board
Ok. You'll need to come along since we need two people to turn the keys to access it.
- The Fire Temple's inner sanctum in Avatar: The Last Airbender could only be opened by five simultaneous fireblasts. A "fully realized Avatar" could do it single-handedly, but it otherwise required five powerful and fully-trained firebenders.
- 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.
- One of many many secure location tropes parodied in a Family Guy "found a new place to hide my porn" sequence.
- 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.
- On The Simpsons two keys were required to be turned simultaneously to drop the "perfect 300 game" balloon at the bowling alley.
- An episode of the animated Dilbert parodies this: while Dilbert and Dogbert are stuck behind a driver at a tollbooth, they engage a dual-keyed weapon to destroy her vehicle.
- The story arc of My Little Pony: Friendship Is Magic's fourth season involves a six-locked chest that appears after the Elements are returned to the Tree of Harmony in the second half of the season premiere. Each of the Mane Six's keys is a memento given by a supporting character when they both learn a lesson about friendship.
- Ben and Kevin hacked the Omnitrix offscreen in Ben 10: Ultimate Alien to ensure he wouldn't turn into Alien X by accident- a Reality Warper with the huge drawback of a conflicted mind that can't seem to agree on any mode of action- without using two keys to access it on the transformation options screen in case the Godzilla Threshold was crossed.
- 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 did happen.
- 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, very quietly, SAC installed these devices, intended to ensure the safety of the free world, and very quietly, they set the combination on every single one of them to 00000000. Very trusting people, SAC.
- Well, 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. These warheads, unlike those for ballistic missiles, can be stored or transported in a functional or semi-functional state and thus may be lost or stolen. For these weapons, the two-man rule utilizes 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.
- Strategic Air Command also had the "No Lone Zone" rule, or "Two Man Rule," where certain locations in nuclear weapon installations required at least two people to be together, always within sight of one another. This was an airtight rule which could not be broken under any circumstances. One instance of this would be in 1968. Two missile maintenance technicians were servicing a Titan missile, when one fell off the platform to his death below. Since the remaining technician was now by himself on the platform, his superiors had to first report a Two Man Rule violation, then report about the fallen crewmember.
- Yes, that actually did happen.
- 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 the customer's. 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.
- At least, this used to be the case. 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.
- Vault doors also have this in some cases as an alternative to time locks, with two sets of combinations and two dials. It has the advantage of allowing the bank to access the inside of the vault, which in one notable case was useful as thieves had tunneled inside the vault(a slight flaw in time locks as there is no way to look inside the vault if someone is thought to be inside, there is absolutely nothing that can be done).
- 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 footpedal.
- 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. Because as Josette Sheeran says; food is gold
- Many main electrical power switches can be padlocked in the OFF position. Devices are available which allow multiple padlocks to be connected to the switch; unless they're all unlocked, the switch cannot be closed. Useful if there might be several people working on the circuit and you want to be sure that it can't be closed unless they all agree. On these, the locks don't need to be removed simultaneously, they just all have to be off at the moment the switch is closed.
- The industrial practice of Lockout-Tagout often includes multi-padlock devices like the one mentioned above. When a major piece of machinery is going to be serviced, you often want to put multiple locks on it so it doesn't start running again unless everyone with a key agrees it is time. The lock will often have a tag attached to it, so you know who to talk to to find out why it is locked-out.