Hmmm, I've never done two briskets. I doubt you could fit two 15 lb briskets on the top rack without getting creative.
I've done a brisket and a big butt, one on each level (brisket top, butt bottom - no pun intended). My bottom grate runs about 15F cooler than my top grate. The brisket was done first, then I moved the butt to the top.
So, if I was going to do two 15 lb briskets, I'd do them on separate racks, then switch halfway through the smoke if I wanted them to finish about the same time. If time wasn't an issue though, you could take the top one off when done, then either crank up the heat or just let the bottom one get done when it's done.