i am doing it with some very small corn snake tubs (the snakes are about 7-8 months old) and i actually just taped a strip of heat tape onto the wall for back-heat. works perfect for me right now. you don't always have to use belly heat
if you DO decide to heat each individual tub, i wouldn't do it the way you drew it in your diagram. i know heat rises, but even if you have a heat tape on the TOP of the tub, i would think it would have SOME effect