Therapy happens (a good therapist at the time of the book, but VERY bad ones in the past, including one who is transparently meant to be Robert Mayer, who wrote Through Divided Minds). Penny's system chooses to integrate/fuse, while Andy's system is utterly repelled and against the idea.

Also, both systems have been through HORRIFIC abuse, and that is described heartbreakingly. Memory work is also described. Penny's system is also in chaos and only somewhat selves-aware, which can be painful to watch.