Idioglossia - or "twin speak" - is a real phenomenon, although experts are divided as to whether it can be considered a real separate language or not. Either way, idioglossia tends to be most advanced and developed in instances where twins are neglected as children and left to speak exclusively to each other, which Fai and Yuui definitely were.
Title: Family Portrait
Rating: PG-13
Warnings: Angst, silliness
Spoilers: EOS, no explicit reference to series events
Pairings: Kurogane/Fai, Yuui/Tomoyo.
Author's Note: Twin AAAAAAAANGST. Serious mood whiplash, honestly. We finally get to hear Yuui's sad past, and how he lost his own twin.
~to be continued...