What's Left of Me: A YA Science Fiction Thriller of Two Souls, Forbidden Existence, and Survival (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. Why aren't they settling? Why isn't one of them fading? 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. Except, she wasn't... 

For the past three years, Eva has clung to the remnants of her life. Only Addie knows she's still there, trapped inside their body. Then one day, they discover there may be a way for Eva to move again. The risks are unimaginable-hybrids are considered a threat to society, so if they are caught, Addie and Eva will be locked away with the others. And yet...for a chance to smile, to twirl, to speak, Eva will do anything.