There are all kinds of technical possibilities. We could advise you better with some context. Is this for a music system, home theater, or both? What are you using for speakers?
In general, I would favor two subs covering the same frequencies. That will tend to smooth the bass response in your room. For a music only system I would recommend setting up the subs in stereo. That would require an external crossover and feeding the subs from your left and right channels. If you don't listen to classical music, ignore that.
Jim