I'd be afraid to use just the lid that comes with the tub as it doesn't seem secure enough to prevent escapes. Generally, the tubs are in a rack system, which makes owning multiple animals very easy. Check out the set ups on animal plastics or boaphile and you can get an idea. I know that some people make their own rack.

I doubt you'll need any additional heat aside from the UTH (which goes outside of the tub). I live in San Diego, and I find that the ambient temps in the tub are fine--but my snake room is pretty warm just from the multiple racks in there. Tubs really make caring for the snakes easy, and the BPs really seem to prefer them.