As long as they are the exact same items (same brand, size, etc) and do not exceed the maximum wattage of the dimmer, you should be able to.
Think about it like this - if you apply the same amount of pressure to the gas pedal of a Toyota Prius and a Chevrolet Corvette, you're going to get very different acceleration rates and final speeds. The same logic applies to plugging in different heat mats (the cars) to the same thermostat (the gas pedal). If you want any chance at reaching the same speed (temperature), you need the cars (heat mats) to be the same too. Make sense?