hey Jon,
last week i cooked 2 13lbers for a work thanksgiving lunch. I smoked them between 266-275 and it only took 4 hours. i was estimating 5-6 hours but at the higher temp they finished early.
So to answer your question...you do not need to worry about the combined temp...1 15lber, 2 15lbers, or 3 15lbers will cook in the same amount of time. but as eman said....it's all about temp. they might be off by 5-10 degrees.
if you cook at a temp of 265-275, like i did, i would say you would probably be done within 5 hours. if you do a little lower such at 240-250 you might add a hour or 2. remember poultry doesn't have as much fast and connective tissue to break down like pork shoulders and briskets.
hope this help and if you have any more questions feel free to ask and the good people of this forum will help ya out.
good luck, good smokin', and post pics :)