Rob, while I do not have an 1100, and someone with one or a Mod will surely jump in, I think 2 butts is going to be your limit IF you can size them right. If you've thought about it, I would discount the idea of cutting them up to fit more in. Bigger pieces and smaller pieces achieve the proper temp at different times, so you'll lose some moisture opening and closing the door. I'd plan on 4 butts, since there will be some loss in weight as they cook, choose 2 a little larger and two a little smaller, run 1 of each together and monitor temps remotely, like with a dual probe maverick thermometer, and pull when you hit 200-203.
Other suggestion is to borrow another unit, but you don't say where you are.
I believe in catch and release fishing. I don't want to EAT 'em all, I just want to GREET 'em all!