It will be the same as cooking one 8 lb butt. It's based on the individual weight, not cumulative.
By the way, to reply back to my original post, I purchased the adjustable rig from theceramicgrillstore.com and it's pretty sweet. I took this photo after laying out the meat to see if it will fit. It's five 8-lb butts. Seems to be plenty of space below the lower rack for the smoke/air to circulate. Smoking them in two days... hope it goes well.