Imo an advantage of using more than one subwoofer is that you can distribute them far apart and asymmetrically such that each is a different distance from the room boundaries. The result is a significantly different room-interaction peak-and-dip pattern from each sub, the sum of which will be smoother than either sub alone.
@secretguy ime it is beneficial to distribute the locations of multiple subs in the vertical as well as horizontal plane, such that you get the aforementioned summed-room-interaction smoothing in all three planes. If you can, I'd suggest raising one sub closer to the ceiling than to the floor, and leaving the other on the floor.
Duke
subwoofer manufacturer