I would not use a pan. I have a 3 rack system (lowest for a pan) and found no need to use a pan. I have fit 2 butts on one rack in the past. Not sure if you will be able to fit both briskets. Perhaps a combo of a Brisket and Butt on each rack.
If you didn't buy the meat yet bring a tape measure or cut a 22" templet from paper and bring it with you to the store.