Hi Dee,
Here’s my take on cooking two butts at once on your WSM. Either method should work fine with just a little intervention.
With both butts on the top rack, the sides on the outer perimeter of the grates will be exposed to higher temperatures than the inside because of the direct heat coming up the side of the cooker. About halfway through the projected cooking time, turn the butts to even out the exposure to the higher temps.
With a butt on each grate, the temps at the top grate may be 15 degrees or more higher than the bottom. If one butt is larger than the other, put it on the top grate. If they are approximately the same size, swap the top and bottom around the halfway point to even out their cooking times.
Let the butts come up to room temperature before putting them in. With the increased amount of “cold” meat being introduced, the cooker’s temperature will drop a bit more and take longer to recover than with a single butt. To be safe, plan on it taking the better part of an hour longer to cook two butts than one. As always, if they’re done early, wrapping them and holding in a warmed cooler will allow you to keep them at a safe temperature for a couple of hours until serving time.
The best temperature I’ve found for pulled pork is 195 degrees internal. At about the point, the butts will appear to “slump” or collapse onto themselves and the bone will offer no resistance when you give it a twist.
As far as cooking times, close to two hours per pound sounds about right for cold weather cooking. Keep the WSM out of the wind as much as possible and it won’t care what the outside temperature is.
Good luck and I’ll let you know where to send any leftovers.
Ken