I use two subs of varying quality in the same room with good results (an overall flat response curve), but to flatten the combined response of the two subs I rely on an external equalizer.

The Behringer Feedback Destroyer ( link ) allows you to equalize two channels (subwoofers) individually to produce a more flat response in your room.

On the other hand, you could go the simple route and just plug them both in and see how it sounds without any external adjustment. ;\)

Jason

Epic 80-800: HG Cherry