Even if the subs are acting in stereo, the important part would seem to be, how they interact with one another. Typical speakers operating above the sub range don't produce the bass nodes which subs do and don't interact with one another as much. It then seems that the only way to integrate two or more subs is to tame the overall effect vice trying to tame the individual effect since any real world application will have to deal with the bass nodes from both speakers. Also stereo subs may produce different signals but since they are LFE they are essentially directionless so the "stereo" effect is pretty much negated.