I wonder how it it will be divided into the two parts
Because the book goes back and forth between childhood and adulthood a few times each. So I can see them following suit, though this might make for difficult pacing.
Or, maybe the first movie will be their childhood, second movie adulthood, almost like a sequel. From the looks of the trailer, this is a possibility, though it could easily be them not wanting to give that away. Because showing all of their childhood in the first film could give away things that are better revealed at the climax of the story.