I had no issues smoking the 8lb brisket, and 10lb pork shoulder at the same time this past weekend, both came out great. I kept my MES 30 at 225, used hickory chips, and a mixture of water/beer/apple cider vinegar in the water pan.
I put the pork shoulder on at 9:30pm saturday night, foiled at 165 degrees, and it finished at 12:30 pm Sunday. I put the brisket on at midnight, foiled at 165 degrees, and it was done at 9:30am. I couldn't believe the brisket finished that fast, and I was actually quite worried that it finished too early to serve at 4pm. I filled my cooler with extremely hot water for 30 minutes, dumped the water, put the brisket wrapped in foil and a couple towels in there, and it was still at 165 degrees when I sliced it at 3:45pm.
That is the fastest I've ever had a brisket finish, and was very surprised that having two large pieces of meat in the smoker actually made them finish faster. Only thing I can think of is, I put the brisket on the top rack, and maybe the smoker was hotter at the top?