agree with all said. it was already said but just to reiterate make sure the fat cap on the briskie is facing down towards the coals to act as a heat shield. you will be eating one dry flat is you do it the other way around especially with nothing under the briskie to shield it from the coals. also, the briskie will probably done first and remember the temp on the grate with the briskie will be hotter than the temp for the grate with the pork shoulder. i have never used a wsm, but i assume it is pretty similar to my uds, that when you put a lot of meat on initially the temp of the cooker usually drops a bit so i would wait for the cooker to get to at least 275, if not 300, when you first put the meat on if you want to have your cook temp at 250 because the cold meat will bring down the temp of the cooker. good luck and post plenty of pron!
Mark...UDS, 22.5 weber knock-off kettle, gasser. GO BROWNS!!!