As Dave points out, there're two different approaches in locating two subs. Any two subs playing simultaneously result in a 3dB level increase, however if the two cones are located very close to each other(either horizontally or vertically), within about one-quarter wavelength, they mutually support each other and the level increase can be 6dB rather than 3dB.
The other approach looks for not an extra increase in level, but reducing room modes for smoother bass throughout the room. For this purpose the subs are widely separated at the midpoints of opposite walls or in diagonally opposite corners.
