T, multiple subs increase bass output and can also result in smoother bass, depending on placement in the room. If they're widely separated, as you're proposing, the increase in output would be the 3dB increase experienced when any two speakers output the same sound. If they're located closely together(within about 1/4 wavelength at 80Hz or whatever highest frequency they're responsible for)the increase in output can be as much as 6dB, because of mutual reinforcement at that close distance.
Smoothing the bass can result from placements as shown in the frequently cited
Harman study by Dr. Toole and his colleagues. Placement of two at the middle of opposite sidewalls or the middle of the front and back walls was best, but diagonally opposite corners, as you propose, were also of benefit in reducing resonances because of the subs being located at points where the phase relationships were opposite each other.
