I have two racks in mine, but only one for cooking, since I use my mini-WSM for very small cooks -- like some wings or a meatloaf.
My bottom rack sits on the indent in the IMUSA tamale pot they the steamer plate would normally sit on. I use that to hold a drip pan.
Being a small space, the mini-WSM seems to be pretty forgiving on cooking grate placement, so I would just work my way down from the top to place two cooking grates. You won't need any room for ABTs, so put that grate just below the top, and place the lower grate to accommodate the stuff you want to put on it.
CD