Gemini, by Sonya MukherjeeRead about the other books on the list.
The key to life for Clara and Hailey’s family has been to stay in their small town forever, surrounding themselves with people who’ve long ceased to be surprised by the presence of conjoined twins. But when talk of the future comes calling and a new guy comes to town, Clara and Hailey are forced to acknowledge they have very different visions of what they want from the rest of their lives. They’ll have to figure out what they want as individuals and how they can both have the future they want and deserve, unless they want to tear themselves apart for good.
Also see Francesca Haig's top ten list of the greatest twins in children’s books.