The outside world is full of dangerous hybrids: two souls in one body. Everyone starts out this way, but it can become very unstable as the two souls age and begin to disagree. Luckily, the citizens of America have worked hard to keep the population pure. In America, one soul eventually becomes dominant until the other fades away completely. And if a hybrid never settles? The government takes care of those unfortunate cases.
Eva and Addie are two souls living in one body well beyond the standard age of settling. Addie is the dominant soul, the one that controls the body and does the talking, but Eva is always there in the backgroud. They pretend they are settled because it's far too risky to let on that Eva is still there. But when someone from school offers Eva a chance to grow stronger and eventually speak and move for herself, Addie and Eva have to make a very difficult and potentially deadly decision.
What's Left of Me  by Kat Zhang is a great story of an alternate universe that feels like it could be real.