I beleve the book splits in two parts because the author wants to show that every one takes on a personality change. But the author wants us to know that Jem changes the most.
                                                                                                                                      ~Jimmy Farkas
The book is split into 2 parts because after chapter 11 their is a time lapse.
Jem is suddenly a teenager and Atticus has taken the case for Tom Robinson.

~Mark Paxton