Tarun, as far as the strength of the response, when two widely separated subs(or any speaker for that matter)play the same sound, the result is an increase of 3dB in combined sound level. As an alternative to getting the 3dB increase each sub could be set slightly lower, presumably resulting in lower distortion, while keeping the overall sound at the same level.
Another advantage of multiple subs beside an increase in level/reduction in distortion is more even room response due to at least partially canceling some of the primary room modes. The
paper prepared by Dr. Toole's colleagues at Harman discusses this and is worth studying. The basic idea is to place two subs opposite each other either at the middle of the front and back walls or the middle of the side walls. In the mid-wall position some modes are at a minimum, and the subs being opposite each other results in partially canceling some other modes.
