So basically what you are asking is this:
1) Can you tear down your muscles multiple times in a day?
2) Can you finish a part of your intended workout later in the day?
3) Is it alright to use the same muscle groups again later in the day?
4) If your workout was originally intended to be Chest, Triceps and Biceps, can you do Chest and Triceps in the AM and Biceps in the PM?
5) Should the muscles be torn down together all at one time, or separately?
Good questions.
I would say "yes" to all 5. (even #5)
I am being facetious of course. I actually do understand what you are asking. You think that since the "healing process" has already started, you are somehow inadvertently delaying it (or preventing it) by working out again- even if it is a muscle group that has not yet directly been worked yet.
Your muscles don't know what time of day it is. If the healing has truly started, I would base my decision on whether or not any soreness had set in by the PM. It probably wouldnt stop me from doing what I had intended to do, but if it did, I would just shoot for the lower body just to make sure I wasnt slowing progress.
I don't really get that anal about anything anymore. I have my size and I keep it with ease. There really are no rules. Do what feels right. If you have the energy to do a split in a day, I say, "just do it" and be done with it.
I can just ramble on and on and on....