Heat is less of an issue than space, you'd seriously have to modify a conventional case to house two power supplies whilst allowing sufficient room for remaining hardware.
The other tricky bit would be linking the 2nd psu to the power button on the case. You can mod a psu to run independently of the motherboard, but I feel it would take some serious work to hook it up to switch both on and off with the case power button.
IMO if you're not happy with the more powerful of the two, sell them both and buy a single higher-spec model.