I do a B&B quite often. Depending on the thickness, marbling, personality, mood, and inclination of the brisket, it might be done before the butt - my 12-lb briskets usually take right at 12 hours, and the butts closer to 16 to 18 sometimes (running at 250 in the lid). Position your brisket on the bottom rack such that you can insert a probe into the middle of the flat from the side access door so that you can check it easily without removing the lid of the smoker.
When you trim off the hard non-renderable fat from the brisket, try to make it in big chunks and save it to tuck under the ends of the brisket where it hangs over the edge of the waterpan. That way, the scrap fat will catch the brunt of the heat rising along the walls of the cooker until such time as the brisket shrinks up enough to pull back from the hot spots. If you trim your butt, save a few scraps of fat to toss into the coals upon occasion. Helps you get that good fat-dripping-into-coals flavor without giving up the control offered by the waterpan.
Just my 2-cents...
Keri C, still smokin' on Tulsa Time (and cooking as Hot Wire BBQ)