Unless one sub is really small, there should be no issue with multiple subs of different manufacture and configuration. I have four subs from three different manufacturers and a couple with different power levels. In my opinion, the only relative constant is that they all can ideally reach the "approximate" equivalent range in the lower HZ(35 HZ and below)level comfortably.
In addition to smoothing out the bass in the room, the other advantage of multiple subs is that you can attain the same SPL levels as one sub by itself but at a relatively lower volume, so you are not driving each sub as hard as you would one on its own.
