Robert, a lot of folks would tell you to put the pork on the bottom and the brisket on top...for at least a couple of reasons: The pork butts are more forgiving, making the top grate the best choice for the brisket to cook more evenly. Also, the top grate handles make it easy to wedge a decent sized packer in place.
Tell ya what though, the best brisket bark I ever tasted came off one that I cooked under a couple of pork butts. Do one or two things to help protect the flat end of the brisket from the higher temps past the edge of the pan though. I've got one of those rib/roasting racks that do either, depending on which side is up. If you use one of those on the bottom rack, you can sort of cradle the packer in it with the ends sloping up a good bit away from the "hot zone" of the bottom rack. Also, if you trim any of the hard fat off, put the pieces (or even a bit of foil) between the end of the flat and roasting rack (or grate if you omit the roasting rack.) The last one I smoked low and slow was like this and it probably weighed about 15 pounds. I've got a butt and a packer in the fridge right now for such a cook in a week or so. I don't plan on an overnighter, though. I'm gonna cook all day at 250-275 for a pulled pork supper and save the brisket for leftovers or the freezer. For smoke wood, I'll use oak, hickory, and apple.
