Your cooktime on two butts should be pretty close to what you expect doing a single. If you can set them up so both sides get fire at the same time and not them stacked "long ways" from the heat you shouldn't have a problem. If you have to stack them going away from the fire, you may need to rotate around halfway through your expected cook time.
Another idea, so you don't have to tend the smoker and pull the shoulder, all the while trying to do a party, cook them the day before, cooler rest them, and pull them into a foil pan (usually 1 butt per pan), then put in the fridge. About an 1.5 hrs before you want to eat, put your oven at 350, add about 1/2in. apple juice to the bottom of each pan and cover with foil tightly. Toss it in the oven and then check it every 15 mins until it's nice and hot, you'll want to make sure you're not burning off the AJ and/or cooking it to the bottom of the pan and toss it with tongs while you're checking.
Once it's reheated, turn the oven to 190-210* and let it ride there until it's time to serve. Allows you to spend time with the Birthday Kid and family.
