Twin Banter

Alexa: You're twins.
Ian: What? No! Really?
Jeremy: Mum should have told us. I feel cheated!
Ian: Duped!
Jeremy: Hoodwinked!
Alexa: They think they're charming.
Liana: Rakish.
Alexa: Roguish.

— Barbie & The Diamond Castle

A Twin Trope. The tendency of twins to banter with each other. This includes Finishing Each Other's Sentences and feeding each other lines, among other things. A common trait of Single-Minded Twins and Twin Telepathy.

This can also be done by Those Two Guys.

A bit of Truth in Television. Since twins are the same grade and age as each other, they spend much more time together than average siblings. They’re also the most likely type of siblings to share a room, especially if they’re the same sex.

Some twins even develop a language specific no one but them understands. However, this happens almost exclusively with toddlers, and they usually stop when they learn how to speak an actual language. The technical term for this is cryptophasia.

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Examples

Anime & Manga

• Kaoru and Hikaru in Ouran High School Host Club bicker regularly, usually in ways that are fully intended to mess with those watching them rather than real disagreements, and they can switch from bickering to talking in sync at the drop of a hat.
• Bakemonogatari's Next Time On sequences are rapid-fire, seemingly random exchanges between Tsukihi and Karen, the main character's younger sisters.
• America and Canada get to do this occasionally in Hetalia: Axis Powers, when they're alone together and America happens to remember he has a brother. Though there's usually a clear winner there, With a notable exception. That one exception is the time Canada made America cry by listing off everything he didn't like about him.
• Goes to show that even sextuplets can engage in it. The Matsuno brothers from Osomatsu-kun frequently finish each other's sentences, squabble, and confuse each other whenever more than one of them is on screen. What makes it even worse is that half the time they don't know who they're picking a fight with since they all look so similar.
• Laila and Leili of A Bride's Story. Their banter is frequently so in synch that it serves as a genuine shock to them when they have a differing opinion.

Comic Books

• Wonder Woman Vol 1: Lillie, Millie & Tillie Heyday speak in unison and finish each other's sentences, though Tillie is more prone to sarcastically banter with her sisters since she's more practically minded. They've actually practiced this to help them with their pranks.

Fan Works

• The cryptophasia version appears in the Mayfair fratello Gunslinger Girl fanfics, where a couple of Agency cyborgs who are twins have developed their own language even though they are adults who already speak several languages. It's pointed out that there's a simple explanation; it's so they can communicate in private in an environment where they are under constant surveillance from the Agency.

Literature

• In The Bridge of San Luis Rey, Esteban and Manuel wind up developing their own language, one that doesn't even resemble Spanish. They are too embarassed to cooperate when a priest tries to quiz them on their language.
• In the Dragonfall 5 sci-fi juveniles by Brian Earnshaw, there were twin alien animals on the eponymous spaceship that work as a living Universal Translator, and do the "finish each other's sentences" version when translating.
• Duumvirate: Dumumvirate bicker duo-style, without sentence-finishing.
• Robert A. Heinlein: Subverted in Time for the Stars. Tom and Pat Bartlett think they have a private language that no one else understands, which they refer to as a "prison-yard whisper". They participate in a study of twin communication, which proves that they can't understand audio recordings of the "whisper". It turns out that they're telepathic. And the telepathy is instantaneous and has no range limit. It works over interstellar distances.
• Also from Heinlein, Castor and Pollux Stone from The Rolling Stones (1952). Castor, 20 minutes older than his brother, often calls Pollux "Junior", while Pollux calls Castor "Grandpa" in return. They insult, prank, and mock fight each other regularly enough that they have a ritual ("Even-Steven!") for formally calling a truce. They also routinely alternate lines when talking to anyone else, and are basically inseparable.
• Fred and George Weasley from Harry Potter. They both can finish each other's sentences, which often includes sarcastic quips. Their Establishing Character Moment features them going back and forth, claiming their mother mixed them up again and causing their mother to do just that.
• Lori's sons Will and Rob Willis (of the Aunt Dimity series) will bicker a fair amount, sometimes with just a simple one-upmanship, sometimes in a larger succession. Their request to visit Skeaping Manor's museum prompts this exchange:
  Lori: Yes, we'll go. If—
  Will: We clear the table.
  Rob: And load the dishwasher.
  Will: And play nicely until bedtime.
  Rob: And go to bed without arguing.
  Will: And promise to behave ourselves in the car.
  Lori (laughing): How quickly they learn.
• In The Migax Cycle, the twins Leafsong and Moonwafer frequently banter with each other, in a mixture of snarky insults and affectionate quips.
• The Night's Blade has twin spymasters Emalley and Edwyn, and their 'creepy little double act'.
• Happens in Diary of a Wimpy Kid with Greg’s neighbors who are twins, Jeremy and Jameson Garza. They created their own language that no one else can understand.

Live-Action TV

• Gem and Gemma from Power Rangers RPM are an extreme case, as it is exceedingly rare for them to individually speak in complete sentences- usually they alternate words or phrases.
• Emily and Katie of Skins were identical twins who were very different in personality, but they had a "twin language" of their own, which was subtitled in the episode where it was used:
  Katie: Sham. [Sorry.]
  Emily: Huh?
  Katie: Katie sham. [I'm sorry.]
  Emily: Why are you talking Twin? We don't do that anymore.
  Katie: Emsy sham doo. [Say you're sorry too.]
  Emily: Emsy sham. [Sorry.]

Video Games

• The Lutece twins in BioShock Infinite. Played with in that they're not so much twins as they are gender-swapped alternate-reality versions of the same person, so it makes sense they can play off each other very well.
• The Frye twins of Assassin's Creed Syndicate. They bicker and trade witty quips, playing off each other's personalities and knowing just what to say to push the other's buttons.

Web Animation

• FUWAMOCO of hololive are a pair of twins (Fuwawa and Mococo Abyssgard) who stream together as one unit, usually with Mococo taking the wheel of playing games while Fuwawa makes backseat commentary. Naturally, they're prone to a lot of back-and-forth, flipping between snipping at each other over disagreements and being uncannily in sync to the point of speaking in unison.

Western Animation

• Jeremy and Ian from Barbie & The Diamond Castle. They even banter in song.
• The Tibble twins on Arthur do this often, competing with each other and often getting into fights.
• Frank and Len from Ruby Gloom.
• In Kim Possible, Jim and Tim Possible have... exactly one call-and-response ("Hikka bikka boo?" "Hoo-sha!", basically "Are you thinking what I'm thinking?" and agreeing to do that thing) phrase that's all we hear of a "weird twin language".

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