Heya! We're the crazy twins who write these books xD
Clio-Sai. The boy, the nice, the charm, the plottwists. Works out the base and skeleton of the storyline as a whole. Clio is the muse of history (and a girl, though Sai isn't).
Calliope-Tarot. The girl, the sharp, the muscle, the style. Takes the bare bones and gives them flesh and skin, so to speak. Calliope is the muse of epic poetry (although Sai's more poetic, really).
As a whole-Aliases for both in one include Mirror Wings, Androgyny and TTT (Tai Twin Team). Any usernames lurking about with Tarot or Tsu in it probably belongs to the both of them.