I think that it might be a better idea to look at "poster printing" for this task.
Scribus is kind of made for situations where you design pages to size. To combine or split pages you would be better off using some kind of impositioning tool. I don't know of any good ones though that can crop/cut pages into smaller parts. But "poster printing" is what it is called when people want to make a large poster using smaller sheets of paper and there should be tools available. /Peter
