i've searched over threads in regards to this issue and found that some members have made this method of heating actually work for them, so i dont see how everyone is saying the temps will get to high and what not. i guess i'll just have to try it out for myself and see what everyone else is talking about because i believe if i have at least a 1" to 1.5" gap for air flow i dont think my ambient temps for the bottom tubs will be to high. i think with having the basking temps set to 92 i might get a high ambient temp of at least 86. i think its somewhat like a heating method implemented by animal plastics where they use "dual side radiant heat". which simply is flexwatt running along the sides of the rack and heating the ambient air inside the tub, here's an email from an animal plastics customer service rep.

"We have recently (within the last year or so)
made our standard heating element 3"/6 watt FlexWatt in the racks when it
comes to belly heat. Another option is the dual side radiant heat which
utilitzes 11"/20 watt FlexWatt that runs down the side walls of the rack
toward the back of the shelves. With that particular heating method, the
focus is heating the ambient air temp rather than focusing on the floor
heat. You still get a temp gradient with your "hot/basking spot" being at
the rear of the tubs with gradual cooling toward the front of the tubs.
Prior to that we had offered heat cable as our standard heating element,
but in recent days we have been experiencing some reliability issues as it
just quits working with no warning or obvious reason." -Ali @ Animal Plastics

so i think if animal plastics can implement this heating method and be successful with it i dont see an issue with stacking my tubs as pictured in my small illustration above. basically its heating the cool side's temps with the heatmat from above which would also eliminate the use of a space heater, saving me a few bucks with the electric bill. also i found a similar thread (http://ball-pythons.net/forums/showt...=stacking+tubs) where a user states they've had success with this method of heating so ima give it a try.