Sometimes a writer wants to include twins as characters but doesn't want to give them different personalities. They're just "the twins", for the most part, and even though they might have different first names, they're essentially the same character. The twins aren't really the Creepy Twins or users of Twin Telepathy; they're written just one character in two separate bodies and have only one personality between the two of them. There's no real difference between the two, and they're generally presented as one consciousness split between two bodies and are never apart.
The opposite of Polar Opposite Twins. If they're on a superhero or villain team of some sort, they'll often be counted as a "single" member (like the Witches 5 from Sailor Moon - which was actually six girls, but two of them were twins). Particularly weird when they're Half-Identical Twins.
A subtrope of The Dividual. Compare Those Two Guys. Often overlaps with Heterosexual Life-Partners.
Ptilol and Cyprine of the Witches 5, who were really one person in two bodies. In the French dub of the anime, they refers as each other ''doubles'', and their names are Émilie and Émilia. Up until they can't agree on how to kill someone, anyway.
There's also a pair of youma under Nephrite who have different powers—dark reflections of Mercury's and Mars'—but otherwise look nearly identical except for color and act as one until they have a spat (over which is doing more to kill the senshi) which gives the soldiers the in they need and the audience a ham-fisted sermon about cooperation.
Despite so little screentime, in the manga Phobos/Deimos, Lethe/Mnemosyne, and Chi/Phi might be like this. Chi and Phi seem to be one-minded in wanting to kill the senshi, anyway.
Jougan and Barinbou, the twin Big Guys from Tengen Toppa Gurren Lagann. They are never seen apart from each other, and while they don't finish each other's sentences, they often do split a complete statement between them, with one taking one full sentence and the other taking the next.
Even before that, it was established early on that Kaoru was passive while his brother was aggressive, both on and off their Host Club "stage." Later on it's revealed that they put on an imitation of this trope to drive people away outside of school. When Haruhi manages to see through this act, they start loosening up, to the point where their true personalities begin to surface.
They're a little surprised by how many differences they actually have; they tended to think of themselves this way a lot of the time, especially after the only babysitter they ever liked turned out to be an infiltrator and reinforced their preference for distrusting everyone who wasn't them. Panic is had on Hikaru's side when he realizes there is a gap forming down the middle of their 'us.'
They did the 'falling in love with the same girl' thing, and their different reactions to the dilemma set off the division storyline. Basically, Kaoru is naturally more mature, and winds up taking care of Hikaru in this regard.
Hansel and Gretel from Black Lagoon. Sure they are presented as slightly different personalities, with different genders, until they switch the gender and accompanying personality between them.
"I'll be Gretel for a while, now."
Ranma ˝ has two sets of Single Minded Twins; one unique to the anime, one to the manga. The anime has Ling-Ling & Lung-Lung, who aren't identical twins (with different hair colours, eye colours, weapons and hair styles), but nevertheless act as functionally one person. The manga has Pink & Link, who act like this when they come to Nerima, but are portrayed as being Polar Opposite Twins in their backstory.
The first movie, Big Trouble in Nekonron, China, has the Lucky Gods Daikokusei and Daihakusei, two Go-playing siblings who are functionally identical. The only difference between them is that one plays white and the other plays black.
They even do a fusion thing where they fight as 'Jasdevi;' it took a while to learn their individual names.
The second season of Slayers has a pair of these; identical twin martial artists who want to be famous pop idols and who get into a fight over what Lina thinks is a book of spells, but turns out to be a collection of lost folk remedies. For added measure, Mimi and Nene are physically identical to Ranma ˝'s Shampoo.
The Tachibana twins, Masao and Kazuo, in Captain Tsubasa. They're also Genre Savvy to the point of using their single-mindness as a game strategy.
The Twin card from Cardcaptor Sakura. They move in perfect symmetrical unison and can only be sealed after being beaten simultaneously, as they're pretty much just two halves of a single entity.
In the increasingly-weird later novels based on the Doom games, the main characters encounter a member of a species of aliens that look vaguely like upright-walking gorillas and are born as Single-Minded Twins. They choose the name, for ease of use of the humans, "Sears & Roebuck".
Mei and Kyū (Para and Dox in the dub) in Yu-Gi-Oh!.
Houzuki and Bonbori from Otome Youkai Zakuro. They're good natured and see Hanakiri Ganryuu as a playtoy, but they are also half-spirit. Good idea not to mess with them.
In Saiyuki, Gonou seems to have believed this about himself and his Half Identical Twin sister, which in his mind justified their relationship. Subverted in that they were canonically matching-but-opposite personalities, and it's presented as unhealthy.
Literal case in Claymore with Alicia and Beth. They were trained/built that way too; the Organization hoped to create a warrior who could becomeanAwakened Being and then come back, something requiring exceptional willpower - accordingly, they decided to design one twin to be the Awakened Being (Alicia) and one twin to be the willpower (Beth). They also weren't the first of the Organization's attempts at this or the last - after them, two trainee twins are seen (they don't last) and, before them, Rafaela and the Abyssal One Luciela of the South, who were initially dramatic failures, but eventually do become Single-Minded Twins as The Destroyer.
In Magical Girl Lyrical Nanoha ViVid, Dieci notes that the first time the twin Combat Cyborgs Otto and Deed have disagreed on anything is whether Corona or Rio will do better in the DSAA Inter-Middle Championship. This disagreement is presumably because the twins each trained one of the girls, and thus chose to favor the one they trained.
Despite being more or less single-minded, they do have distinct differences, the most obvious part being their appearance. Otto is a Bifauxnen and Tomboy who works as a crossdressing female butler, while Deed looks and acts much more feminine and wears feminine clothes. Both were stoic and quite before their Heel-Face Turn, but both mellowed out at the same degree, while retaining their polite personalities; and sometimes, they even wear (feminine) twin outfits. Another obvious difference between them is their Inherent Skill. Otto's Ray Storm shoots lasers, and Deed's Twin Blades is a pair of Dual WieldingLaser Blades.
In the anime version of Trigun this trope is referenced by the young Knives: once positively, with regard to one of them being bullied, and once negatively, after he gives himself a haircut: "if we stay the same, we won't have any individuality."
Centaur no Nayami has single-minded triplets in Manami's little sisters Chigusa, Chinami and Chiho. It's to the point where they're typically called 'Chi-chan', as though they really were one person.
In A Certain Magical Index this is portrayed both more literally than normal and more expansively than normal. There are just over ten thousand Misaka clones left out of an original twenty thousand and they're all referred to both as the Sisters and as the actual sisters of Misaka Mikoto. Though some characters view them as individuals, it would be more accurate to stay each one is a unique unit, all of which are an addition to the overall Misaka network. So in truth they really do all have one mind and they are constantly in mental contact, so when multiple Misakas are present they tend to finish each others sentences without hesitation.
Bob & Ray had a pair of recurring characters named Clyde and Claude McBeeBee, non-identical twin bandleaders who went everywhere together and always spoke in unison.
One comedienne specifically cites and mocks this trope. "I have a twin brother. People are always asking me things like, 'You're twins; can you hear each other's thoughts an' stuff?' 'Yeah. Oh, hang on...he thinks you're an idiot.' "
Ladyhawk, a supporting character(s) in Spider-Girl, is/are a set of twins sharing the same superhero name and identity (so they can operate at full-time for a single hero while still maintaining private lives). They are implied to have distinct personalities, but since their screen-time is so limited, the only observable difference is that one of them is more experienced.
Now that one of them has been forced to retire after being injured by the Hobgoblin, they seem to have become more like Yin Yang Twins.
The Stepford Cuckoos (Celeste, Esme, Mindee, Phoebe, and Sophie) in the X-Men family of books, are identical quintuplets and form a "Five-in-One" telepathic mind. Later events show the "hive" mind splintering, leading to the deaths of Esme and Sophie. Much later in Uncanny X-Men, Celeste dyes her hair and starts dressing differently from Mindee and Phoebe (who are pretty upset about it).
In Robin Year One, Two-Face employs a pair of these, identical suits and finishing each other's sentences included. Given Two-Face's gimmick it's peculiar that he doesn't have them more often.
Though never explicitly stated (it's implied to be the result of a string of freak coincidences), Batman villains the Trigger twins. They first met when they both attempted to rob the same bank, only to discover their exact resemblance to one another. Ever since, neither of them has ever seemed to come up with an idea the other hadn't been thinking of as well.
The Thompson Twins in Tintin, who are not technically twins but are virtually indistinguishable - the only difference is their moustaches. Furthermore, in the original French, they were known as Dupont & Dupond, which are even pronounced identically - this was kept up at least to some extent in most translations, including English, where they are known as Thompson and Thomson.
In standard English Thompson and Thomson are pronounced identically as well, by anyone who isn't consciously trying to demonstrate how they pronounce them differently.
Ken & Kyle Katayanagi, the 5 and 6 evil exes of Ramona, from Scott Pilgrim. While we can't see much of them through volume 5, as they usually send robots to attack Scott, they pretty much seem to think and talk at the same range, finishing each others sentences at times.
An author going by "persian85033" has produced an X-Men: Evolutionfanfic involving, may God help us, the Single Minded Dectuplets Maria Xoaquina, Maria Xitlalli, Maria Xaviera, Maria Xacinta, Maria Ximena, Maria Xiomara, Maria Xochilt, Maria Xosefina, Maria Xulia, and Maria Xamila, who literally refuse to be separated even for the length of a single school period: the entire school has to be rearranged so that they can be in the same room for every class.
In Harry Potter and the Methods of Rationality it's stated that Fred and George have almost identical thoughts if they're given the same information. They only talk to each other to share something the other doesn't know about and on the very rare occasions that they disagree with each other they feel like something has gone deeply wrong. Apparently this is normal for magical identical twins and in times past families would actually kill one of the twins at birth.
In Error Of Soul, another Harry PotterFan Fic, Fred and George are continuously synchronised, so they're unable to remember who is who from day to day. The central concept of the plot is this gradually happening to *Harry and Hermione*, a situation that normally ends with the unfortunate soul-bonded pair trying to reach opposite sides of the planet.
The Corsican Brothers, as portrayed by Cheech and Chong. (Also, originally, as portrayed by Douglas Fairbanks, Jr. (1941), which Cheech and Chong were riffing off.)
In The Prestige, "Borden" is actually a pair of twins who have spent their whole life letting everyone else think they are a single person; whenever they appear together in public, one is heavily disguised, and they switch roles as necessary. The only flaw in the plan is that they fall in love with different women.
The Twins from The Matrix Reloaded, probably justified seeing as how they are computer programs.
Although it's entirely possible they're doing this deliberately to mess with Mal.
Numbers 3 & 4 from the movie 9, both of which are very skittish, are very curious about the world, and never (audibly) talk outside of their own eye-flashing language.
They also have projector eyes, or at least one of them does.
Skids and Mudflap from the Transformers: Revenge of The Fallen, both portrayed as rednecks trying to be gangstas. Skids, however, is somewhat more aggressive and intelligent then his brother (and also has a slight case of hero-worship for Optimus Prime), even though Mudflap is probably the better fighter and driver.
In Dude Wheres My Car, the twins that the heroes are dating spend all their screen time talking quickly and alternating with each other, and occasionally say the same thing in unison. An odd example as the actresses who play them, while they share a certain vague resemblance, are not identical twins or even related.
Fred and George Weasley from Harry Potter, at least superficially. See here for an analysis of their differences.
Rowling specifically envisioned slightly different personalities to the twins. Fred is braver and more outgoing and is the "leader," while George is more clever but meeker. For example, Fred is the one who initiates just about every hi-jinks they ever pull, and the one Ron complains about the most. The difference is very subtle due to their limited screen time, and their enjoyment of using this trope on their mother. To her credit, many found it surprising that Fred was the one who died, stating that George's personality made him the more obvious victim.
Sam and Eric from Lord of the Flies. Throughout the book, their names evolve from "Sam and Eric" to "Sam 'n' Eric" and, finally, "Samneric", cementing them as one person. The only time they talk independently is after their camp is raided showing how effed up everything is getting. Even their description supports this idea - their skin looks stretched tight across their bones, as if they don't have enough skin between them.
The Duumvirate actively tries to fulfill this trope. Telling them apart is considered a major challenge even for their close friends.
In the Spider Robinson book Lady Slings the Booze, Arethusa is literally a pair of single minded twins, two women who from birth were telepathic with each other to a constant degree, combined with some weird superstition in their parents, caused them to grow up as a single person in two bodies.
The Twins from Eragon. They don't even have names. Ajihad says to Eragon "I would tell you their names, but they have none." They act pretty much the same, looking alike, acting alike, and having the same loyalties.
In another rare use of the 'One person, Two bodies' ploy, Miss Level, a village witch first met in Terry Pratchett's A Hat Full of Sky (Discworld), was born with two bodies, and people assumed her to be a set of twins, "then they thought [she] was evil." She lives a very prosaic existence for someone with two bodies. She meets an unusual end: when only one of her bodies is killed, she survives, though is weakened from the blow, and eventually finds that she still remembers how to use two bodies, and thus, being a witch, gains basically telekinetic abilities.
Ursula K. Le Guin's short story Nine Lives featured 10 clones who were essentially one being. When nine of them died in an accident, the survivor considered himself "nine-tenths dead" and nearly lost his will to live.
Ian Mc Donald's Desolation Road (set in the far future) had an incident with two clone-assassins, AlphaJohn and BetaJohn. They were two clones who were one being, with perfect telepathic connection, having the advantages of sharing all their senses. The victim realizes that he can shoot one of them, and they'll both die. So he does that.
In the Old Kingdom trilogy by Garth Nix, there are the twins of the Clayr, Sanar and Ryelle. They speak in synchronization and it is noted that their souls are bound around each other too tightly for even magic to treat them as separate beings. Though they do have some different traits; Ryelle is a better paperwing flier.
Used on two separate occasions by David Eddings:
In The Belgariad, Beltira and Beltika are like this before they become immortal sorcerors, and have both telepathy and millenia to become even more familiar with each others' habits and thought patterns. It's even effectively weaponised at one point, when Belgarath realises that their ability means they can cross-corrolate two sets of prophecies (one detailed but utterly random, one sparse but in chronological order) to find the similarities and make a comprehensive whole.
In Regina's Song the title character and her sister Regata are this, until the latter is raped and murdered, where they effectively become single-minded twins in a single body. Kind of. It's complicated.
Used in some form in A Breath Of Snow And Ashes in the Outlander series, with Jo and Kezzie. Namely, in that they end up mutually devoted-- and married to-- the same woman, and there's a bit of a kerfluffle over whether it's "really" bigamy or not, if twins can be taken as the same soul shared between two bodies. Yeah. It's a bit weird.
In Whispers, by Dean Koontz, the heroine was almost raped and killed by a famous man before killing him herself. He couldn't have done it because he was away. As you can guess, it was his twin. The other comes back and talks about how she killed him. Turns out their mother was impregnated by her father and gave birth to twins. After some time, the stress drove her insane and she believed they'd been concieved when a demon raped her. To conceal them, she forced them to be one person, and locked them in a cellar with hundreds of horrifying, huge cockroaches that would crawl all over them when they didn't act or think as one. As a bonus, she made them believe they had demonic penises, and that they always had to hide them from everyone else. Now that they don't have each other, their only "outlet" is with the only other person who knows - Mom. She's dead - but he thinks she's coming back as different women. Shudder.
Mark Frost's The List Of 7 has Larry and Barry, identical twins and professional thieves. No one knew that they were indeed twins, so they posed as the same person. One of the brothers would appear in a pub or restaurant making a big show(one of the brothers liked to sing while the other liked to recite ribald limericks) while the other brother went to work cleaning out someone's flat. If one of the brothers was suspected, he could produce any number of witnesses to give him an alibi. When one of the brothers received a scar from a bullet wound, both brothers grew beards to cover the scar and maintain the fiction that they were the same person. Ultimately they were found out by Agent to the Crown Jack Sparks who pressed them into his service in exchange for a pardon for their crimes.
Ani and Tish act like this, but in reality Tish is almost three years older, they have different mothers, and Ani is almost a full faerie where as Tish is practically mortal.
Ritva and Mary Havel of the Emberverse begin very much like this, but in the course of the second trilogy they show/develop somewhat more distinct personalities.
In The Vampire Files series, the Ruzzo brothers look and act so much alike, and work together so closely, that their fellow gangsters refer to them in the singular as well as the plural (e.g. "Tell Ruzzo to get their butt(s) in the car, already.").
Flora and Fauna, Peter and Fudge's second cousins in the Judy Blume book Double Fudge. The girls dress alike and sing together. To make things worse, the girls finish each other's sentences.
Eustace and Claude from the Jeeves and Wooster series. At one point, when they're supposed to be heading to South Africa, they both independently fall for the same girl and independently leave the liner before it departs. They're then shocked to find each other when they both go back to Bertie's flat.
Cersei and Jaime Lannister from A Song of Ice and Fire think of themselves this way — Jaime is convinced that "If I were a woman, I would be Cersei," and Cersei says she only feels whole during sex with Jaime. Their POV chapters, however, reveal that they're wrong about this.
The Gowder twins in The Stranger House don't fulfil the 'never apart' part of this trope, but they are not only indistinguishable but treated as entirely interchangeable by the other characters. They are known as Laal and Girt:note meaning little and big whichever twin you are talking to is Laal, and you never meet Girt (presumably when they're both present Laal is whichever you are addressing at that moment). No-one knows what the twins think about this. They're... odd.
Anything You Can Do... has a rather... scary yet different subversion. When Mart is two, his father brings him to work, 'just stopping by'. A bad accident kills the father, leaving most of Mart's nervous systemnote everything but most brain and some spinal function severely damaged. The doctor recommends multilevel therapy, but the mother doesn't follow through. Mart shares a psychic link with his twin Bart. Mart's condition means their minds can't balance out; Mart does not have the wealth of experience needed. Over the years, Bart's mind essentially copies over to Mart's.
The Age of Fire series has SiHezethant and Regalia, rare dragon twins born from the same egg, who are constantly commented on being so similar in appearance and mind.
Mahou Sentai Magiranger's Nightmare Sisters aren't even identical, but almost all of Mea's lines amount to repeating the last one or two words of whatever Nai just said. (Of course, they're both halves of a vampire queen who split herself because she got lonely.)
Power Rangers RPM has Gem and Gemma, who don't appear to be capable of independent thought (or speech; the page quote is probably the only time they aren't written as if only one person was talking, with Gem delivering the first half of the sentence and then Gemma taking over for the second half). For all intents and purposes, they are one person in two bodies. No explanation is made of this. In fact, it's not even commented upon.
At least, not initially. Six episodes after their first appearance, they finally begin to be split up (though usually not by their own choice) and are given individual plot threads and slow-going characterization. However, it's doubtful they'll ever lose the singleminded-ness, except on the rare occasions they disagree on something (when they do disagree, it's generally about romance, with Gem being overprotective of Gemma).
Averted in Star Trek: Voyager. When the oft-mentioned but rarely-seen Delaney twins from Steller Cartography finally make an appearance, one of them is attracted to Harry Kim, much to his annoyance as Harry prefers her sister. When a nonplussed Tom Paris points out that it shouldn't make any difference, Harry proceeds to list a number of character differences between the two, ending with "that cute dimple on her right cheek." In their one appearance, while playing twin henchwomen of Dr. Chaotica in the Captain Proton holonovel, they act distinctly different, one more assertive than the other.
Battlestar Galactica: The horde of Number Eights that confront Boomer in "Kobol's Last Gleaming" finish each other's sentences to creepy effect. Averted with Boomer herself, and Athena and a handful of other Eights that stand out. Averted by the Sixes constantly but played straight by the other models constantly.
It's eventually revealed that disagreements are settled by each of the twelve models voting as one, and Boomer causes quite a stir when she votes against all the other Eights.
7th Heaven with Sam and David. One wonders if the show runner has ever met real life twins.
The series Picket Fences had deputy Kenny briefly date a pair of Separated at Birth twin sisters whose bond was so strong it drew them together. In their first appearance, one sister had to pull her car over to orgasm while the other sister was having sex — and this was before they even met! After they do meet, they insist on dating Kenny jointly.
The Cousins in Breaking Bad, reinforced by the fact that they barely speak.
In his book I Am a Strange Loop, philosopher-scientist Douglas Hofstadter describes an Alternate Universe called "Twinwirld", in which (almost) everyone is a pair of Single Minded Twins, called a "pairson". (Single births are called "halflings" and considered unbelievably pitiful.) This is an analogy for the two hemispheres of the brain.
Alpharius and Omegon from Warhammer 40,000 are described as "two bodies with a single soul".
Dungeons & Dragons featured a race called the Dvati who were based on this trope. Each "individual" Dvati was a pair of empathically linked identical twins. They literally only have one soul(and by extension one set of hitpoints, divided evenly) between two bodies.
Kat Brokensoul, from the Werewolf: The ForsakenNight Horrors book, is actually two werewolves operating almost seamlessly as one person. In fact, their father specifically orchestrated it so that, legally, only one of them existed for... some reason. They even have a rite that lets them swap scents and employ a limited sort of Twin Telepathy, which didn't help their becoming The Dividual.
Subverted in the stageplay version of Peter Pan. Two of the Lost Boys are twins, but Peter had never seen twins before they showed up and wasn't sure what to do with them. By the time the book begins they are simply each called "Twin" and are treated as one person. One is never without the other and they finish each others' sentences. Lampshaded when only one has a dream and the other says not to tell Peter because they 'didn't think they ought to have different dreams'.
Timmy and Tommy Nook from Animal Crossing, who both say the same lines, with little gray words at the end representing the other one saying it at the same time.
For Basch and Gabranth/Noah it's quite complicated, as their different personality might be due to different "environment" they faced.
In the backstory for Xenogears (as revealed in the games Perfect Works book) Miang's incarnation during the Zeboim era is as a set of identical twins, making this another example of the "one person two bodies" variant
Gwendolyn and Charlotte, the twin daughters of Rose Somerset from the Ravenhearstgame series, are a Creepy Twins example, speaking in synch with one another and dressing identically. It's downright shocking to see them separated, even temporarily, in Escape From Ravenhearst. Possibly an enforced example, as the girls spent most of their brief lives and unhappy existence as ghosts being browbeaten by Charles Dalimar, who wanted his stolen "family" to conform to his notion of doll-like, carbon-copy perfection.
The Lutece Twins finish each other sentences, always travel together and are rarely seen far apart. Subverted in the fact that they are actually the same person, but from different dimensions.
In Schlock Mercenary, T'Chukk is from a race of bicameral lifeforms; he's a single being in two bodies, synced by radio. A bit of hyperspace communications augmentation, and he's able to be his own wingman in the crew's fighter craft wing.
In Linburger, Firne are a single person in two identical twin bodies that change their sex organs with the phases of the moon (but always have boobs).
Zig-zagged in Grrl Power. Harem has the ability to create up to five duplicates of herself, and they all share one consciousness. But Harem has personalized each one, with different hairstyles, tattoos, piercings, wardrobes. The cast page specifically states that for each different body, she affects different personalities, but it's largely a farce. In the "Dabbler's Science Corner" comic, she specifically states that there's no "this one" or "that one" for her bodies, there's just her. In summary, Harem is one person that has the potential to create a Single Minded Quintuplets situation, but goes out of her way to not do so.
Which is subverted whenever their plans (frequently) backfire on them. The female twin is shown to be easily angered and volatile while the male twin is calmer, submissive, and more sensible, but becomes dominant and protective when his sister breaks down when pushed far enough.
Marge's sisters, Patty and Selma, started out this way, but diverged from each other over the seasons. Selma craves companionship, having gotten married several times and eventually adopting a Chinese girl, while Patty was earlier described as not liking to be touched, and more recently came out as a lesbian.
Notably averted in Invasion America, which had Sonya and Simon, a pair of psychically linked twins with super-powers which were stronger when they touched each other... and still gave them different personalities and independent character arcs.
Bill and Ben in Thomas the Tank Engine (and to a far lesser extent, Donald and Douglas, although they are more independent of each other).
Skarloey and Rheneas. Both started out as young and hot-tempered but at 140 years old they have mellowed out quite a bit.
Rumored to be the case with I and Am, a brother team of bounty hunters who appear in the Samurai Jack episode "The Princess and the Bounty Hunters". Whether this is true or not, the one of twins often finishes a sentence that the other starts (and they often rhyme their words) and as proven near the climax of the episode, they can communicate with each other telepathically.
In Batman Beyond: Return of the Joker, Dee-Dee/The Deeds/Delia & Deidre Dennis are twin sisters who may as well be two halves of one person. Besides looking and talking exactly the same (both twins are rapidly and amusingly voiced by Melissa Joan Hart), they move and fight in incredibly effective teamwork. Just goes to show how devastating it would be if villains didn't attack the hero one at a time...
In Justice League Unlimited they return for an episode set in a time travel corrupted future where they're given the power to duplicate, making even more copies with the same single mind.
They have been known to address one another as 'Dee Dee'; as in "You think so, Dee Dee?" "Oh yeah, Dee Dee."
Walter and Perry of Home Movies aren't related, but they are never apart (and almost always holding hands). Their less-than-totally-innocent relationship makes them essentially one character.
Jetfire and Jetstorm from Transformers Animated. It's made all the more literal after they are badly injured and implanted with Starscream's programming during an experiment in allowing Autobots to fly. Besides being able to turn into jets, they gain the ability to combine into a single form known as Safeguard.
Tad and Chad on The Fairly Oddparents are a variation. They aren't related, but other than different skin colors they could be twins.
Katie and Sadie from Total Drama Island are another case of unrelated friends who act almost exactly alike and are horrified at the thought of being separated. Interestingly, Fanon often tries to interpret or develop them to be more separate, while the show passed up a perfectly opportunity to do so (between Katie and Sadie's eliminations}.
Superjail!'s very own Twins fall under this trope most of the time they appear, and that's only one of the many strange parts to them.
Tek and No from Foot 2 Rue are definitely this, usually being referred to by their portmanteau name the Teknos, and being completely identical. They start becoming a little more individual in season 3, but are still impossible to tell apart.
Whittany and Brittany Biskit in Littlest Pet Shop (2012), who almost always agree with each other, and when in a musical sequence, always sing in unison unless complementing each other's lines. However, there have been a few moments when the two are separated, where it's shown that each of them is intentionally trying to act like the other one to avoid rejection from her.