2 different drivers (types, models, brands, sizes ETC) playing the same frequency will not reach the same, and therefore will cause cancellation. it my sound ok when you do it, but having one on its own, in a decent box (which you probably dont have if your thinking of mixing subs) will outpound the 2 non- matching subs, every day of the week.
there is one way you could. You would have to build some custom crossovers and seperate the frequencies each sub works on. But whats the sense of that?