I thought of that too - and put the hot one on the bottom - temps stayed the same though. (with no duct hooked up at all) The hot one, even on the bottom, runs ~20c hotter than the other one. I also tried only running the hot one by itself and the temps are still at ~100. Room temp is below 20c all the time (there's alot of airspace and concrete floor so that room is quite cool all the time).
In that case, I'd say your having a cooling problem with the hot unit. It could be:
1) A dead or underperforming fan (what speeds are being reported?)
2) Dirt on the boards clogging the heatsinks or otherwise obstructing air flow
3) Missing or lose heatsinks
Suggest focusing this conversation on that hot unit for now.
If this unit is still under warranty, you MIGHT violate that by opening the box. If its not:
A) Take off the fans and plug them into a fan port on any CPU motherboard and see what happens.
B) Open the case and pull the hash cards to visually inspect them