You don't want to stack meats when smoking. They will not cook evenly and wouold require much higher than normal grate temps and frequent rotations to even come close to getting them cooked in a reasonably safe amount of time.
I've stacked them into foil-tented baking pans for the brase/steam, and was not happy with the out come. The bottom will cook while the top just lays there in limbo waiting for something to happen.
Get the rib racks as mentioned above, but keep a space between the slabs of ribs, or they will act the same as if you stacked them. If it's a 6-bar rib rack, you can only use 3 spaces (every other space) in order to get evenly cooked slabs of ribs. I have overlapped my rib racks from end-to-end so that I can get a few more spaces between the bars on the same cooking grate.
Eric