Putting one sub in a corner will excite all the room nodes, good and bad. Adding another sub near the same corner will increase the output, I believe it is 6dB, and I assume those nodes will also be even more increased.
Depends on what your goal is, for me I want a smooth frequency response in all locations of the room, and still getting great output that is not boomy as a result of exciting the nodes. This can only be accomplished by placing the subs opposite of each other, either mid wall or opposite corners. Doing this helps cancel out the nodes, giving you a flatter/smoother response.
Maybe you should try opposite corners.
