I have dual SVS subs. I put them on opposite corners next to my ML speakers. In my room, I had to put one sub 180 degrees out of phase so they did not cancel each other at my listening area. In my manual, SVS said that if you can put them in the same corner, you will most likely not have to deal with them cancelling each other out and that if they are seperated, your total output will be lowered by 3 dbs.
I really like having them seperated even with that drawback. I run stereo subs for my music playback! My left channel first goes into my sub and then out to my speaker and the same for the right channel. With two subs, it just seems more seemless in presentation.