One sub won't cancel another out if you have two of them, so long as they're properly wired to the same amp and are in identical enclosures, or the same enclosure.. provided the subs are identical.. when you mix and match subs, that's when you run into issues.. that and when people wire them incorrectly.
As for the best place to put your sub, I tried putting mine (x-230) in the corner, and it was loud.. but the walls and desk only enhance tones around 55Hz+, which aren't very low tones..
I found the sub to sound best with the subwoofer itself (the side of the woofer with the grey grill, on the x-230) aimed at my seating area and the port firing away from the wall.. this left the desk and one wall to enhance the bass to a point, but not where it makes it sound muddy and loose.
-mak