In your situation with only two snakes, I wouldn't buy a rack at all. I would keep 2 separate bins on a single strip of flexwatt, ran with a Herpstat. Get velcro and glue it on the lid and tub to hold the lid on, and/or binder clips so that they are 100% escape proof.
If you buy a rack now, you're going to be paying to heat all of those empty tubs for 3-4 years. That will REALLY add up. Unless AP racks come with individually wired shelves, but I don't believe they do.