Quick question about tubs

Ok so I got my flexwatt and my thermostat and everything is hooked up. My question is can one tub be placed on top of the other or will that make it to hot in the bottom cage?

I've seen people stack tubs on top of each other but with about an inch worth of clearance in b/w each tub. I've seen the clearance accomplished by using a long inch high strip of wood to separate the two tubs. If you just stack them on top of each other you will definitely see them getting mighty hot might quick.