I'm aware of at least two schools of thought on the subject.
The first would recommend placing one sub to the extreme right of the listening position, and the other to the extreme left of the listening position. This works with the ear/brain system to produce the greatest sense of hall ambience.
The second school of thought would recommend placing the subs as asymmetrically as possible, so as to maximally stagger the path lengths from each sub to room boundaries and then on to your ears, resulting in smoother bass over a large area. A low crossover frequency and/or steep slope low pass filter is beneficial in this application. This is the technique I use. In my room one sub is behind the right speaker and near a corner, and the other is along the left wall slightly behind (and much closer to) the listening position. Ideally, one of the subs would be elevated off the floor also.
Duke
The first would recommend placing one sub to the extreme right of the listening position, and the other to the extreme left of the listening position. This works with the ear/brain system to produce the greatest sense of hall ambience.
The second school of thought would recommend placing the subs as asymmetrically as possible, so as to maximally stagger the path lengths from each sub to room boundaries and then on to your ears, resulting in smoother bass over a large area. A low crossover frequency and/or steep slope low pass filter is beneficial in this application. This is the technique I use. In my room one sub is behind the right speaker and near a corner, and the other is along the left wall slightly behind (and much closer to) the listening position. Ideally, one of the subs would be elevated off the floor also.
Duke