Let’s see now….. 18 pounds of butt times a 40% loss factor still leaves about 10 pounds of finished pork. That sounds like about the right amount for snacking on while watching the game!
Since both butts are roughly the same size, swapping their positions a little after your projected halfway point would help even out their cooking times. Those are big butts, but they may also shrink enough in size toward the end of the cook to where they both could possibly set on the top grate if the lower one has some catching up to do.
Allow yourself plenty of time, relax, and have fun. You can wrap them and hold in a pre-heated cooler if they get done early.
Sounds like you’ve got a good plan. I’m on my way now to Sam’s to pick up a couple of butts for tomorrow’s overnight cook. Have a good weekend.
Ken