Cipher

This is a book about two twins named Siva and Cipher, who since entering high school have taken turns going to school and work as a model/actor under the name "Siva." When a girl at school finds this out she is not given a reason, but instead a two week basis in which she lives with them to try and be able to tell them apart without asking. At the end of the two weeks if able to tell the brothers apart, they have agreed to tell her the reason why they switch off. <em class=de-emphasized>(from manga)</em>

Synopsis from Anime News Network