I have used 2 different kinds of subs hooked to the same amp (same ohm loads). While it does get loud, it definitely doesn't sound good. Each sub hits differently and having 2 different bass notes fighting eachother is not the greatest listening experience.
I mean I could picture someone using maybe, like 2 10's for punchy bass, and a 15 for the deep bass, if it was all hooked up by someone who knew their shiet, but I seriously doubt that would be practical for anyone else. Too many things would have to be perfect for it to sound right..