I cooked two ( 11 lbs) on thursday.
They took just about 3 hrs. One was honey brined and the other was cajun with the help of Jim Minions injection recipe ( Zaigaraines, butter, garlic and onion oil)
They were the perfect size for the WSM as I was able to use V-Racks on both. Started both breast side down for the first hour and flipped them over and switched levels. At the end of the second hour switched levels again as the upper grate appeared to be significantly hotter.
Both were excellent. It takes longer to cook two, but not much. One thing to be aware of, is the temp. My Trend was showing 225 for almost the entire cook, but the portable inside was confirming 340, the entire cook. There's very little room on the top, so even if the thermo is not hitting the bird , it's so close that I think it throws the readings off.
Followed Chris's reccomendations with charcoal, ..one full lit webber and one unlit over top, although I added ywo additional handfuls to take care of the extra bird.
WILL