If you are crossing over low enough, then you don't need two subs for stereo as Faberryman and others have pointed out. But two subs often gives a better result - the main reason being that you can get smoother response. Some subs use two drivers, one on each side, and their separation by a foot or so helps to get a smoother response. It is just that each driver will create room nodes and anti-nodes in different spots and (hopefully) cancel each other out. The downside is that set-up requires even more patience and time than with one sub. From experience you will very likely get a significant improvement with two subs, but two Stentors is a big investment in the bottom octave.
