Virtually all CD's are mono below 80 Hz, so I doubt there would be a significant audible difference between dual mono subs and dual stereo subs.
I do believe that dual subs offer significant room integration advantages over a single sub.
From my investigation of the subject, there seems to be two well thought-out schools of thought on dual subwoofer placement. One holds that the ideal is to place the subwoofers to the extreme left and right of the listening position. This maximizes the interaural time difference and thereby the sense of spaciousness that the subwoofers convey.
The second school of thought recommends what could be described as highly asymmetrical placement, to maximally stagger the path lengths from each sub to the room boundaries and the listening position. This results in the smoothest bass, and is the technique that I use most