Only very general spoilers ahead.
The split between the innies and outies seems to me to be a remarkable demonstration of 20th century Swiss psychologist Carl Jung's concept of the shadow. I think it's most evident in Helly and Dylan, yet there's definitely a case to be made for other severed characters.
The ego is the person you believe yourself to be, it is the you that says "I am". The shadow is the aspect of yourself that the ego denies. In my view, the Outies represent the ego, while the innies represent their shadow sides.
The things you deny can be emotions, e.g. you are reprimanded for expressing anger as a child, and you learn to suppress that aspect of yourself. You eventually