What's Left of Me (The Hybrid Chronicles, #1)

Eva and Addie started out the same way as everyone else, two souls woven together in one body, taking turns controlling their movements as they learned how to walk, how to sing, how to dance. But as they grew, so did the worried whispers. The doctors ran tests, the neighbors shied away, and their parents begged for more time. Finally Addie was pronounced healthy and Eva was declared gone.