It is indeed confusing but it is a very common dissociative identity disorder. And don’t, it doesn’t is linked with genetic traits, is especially a scape route that helps the host with their issues. But, is important to say that Yuki and Yuuta aren’t the same person. Yuuta is a child that appears to take care of the body when Yuki doesnt feel like it. It can be triggered by some sort of events (like when he almost died) or they can just pass the command of the body when they want to. That explains why Yuki coudn’t come back for some time after the traumatic event, because he was “sleeping” and recovering, while Yuuta (the trauma holder n protector) take care of the body, but when Yuki got better, he could come back and share the control with Yuuta (≧∀≦)
For those who r confused bout Yuki/Yuuta’s condition, it is called dissociative personality disorder. It is a real disorder, where the host (original personality) starts to create a sistem os multiple personalities to overcome traumatic events. They start with a trauma holder, that in this case is Yuuta, that will help when the first personality is having trouble to handle hard situations.
With time, they can create infinite personalities, I already saw systems with 50 personalities in 1 body. Normally they create 3 separate persons, the host, the trauma holder and the protector!! ヾ(❀╹◡╹)ﾉ~