I did my scene list of the first two thirds of the existing book - the one I'm going to split into two - and I realize that I just might be better off splitting this book later. Which means I may have less work to do.
The problem was this: When I did my read through, I found the first five chapters to be too skimpy. They really do just hop across the highlights of some scenes. Then the next five really pull together to a climax that I figured was the end of the first book. The five after that drag on too long, but end at the other possible ending point for the book.
What I discovered was, on outlining them, that I could seriously edit the last five chapters, and plump up the first five by about the same amount... and I really don't have to make major structural changes. Some of my plans for Henry's story will help in the first five chapters, but I don't have to change his story significantly. As a matter of fact his story breaks in a much more satisfying spot - something of a cliff hanger really, but that's okay for a secondary story line, I think.
And Rocken's story doesn't change at all, because he doesn't figure into those slow five chapters at the end, except for one glowering moment, seen from afar, that fits better at the end of the first book than at the beginning of the next.
Tomorrow I figure out the major plot points, and what would be different about it if I go with my new idea of where it ends.