Yes, put the butt on top and let that butt baste the brisket. Other than that, just keep an eye on temps. Brisket and butt should both reach about 190 internal. I'm not much help on how long it might take those sizes of cuts to reach nirvana, though, because when I do brisket it's usually a 12 - 14 pounder, and my butts are usually in the 7 to 8 lb range. That being said, though, in my experience when doing a large brisket and butt, my brisket is usually done before the butt. Both will benefit from a tightly foiled and wrapped rest in a warmed ice chest before carving.
Prepare to be inundated with compliments. How are you doing the butt? The "renowned Mr Brown" recipe, which can be found on the "Cooking" link on this site, is an excellent approach.
Keri C
Smokin on Tulsa Time