Glenn, sure two widely separated subs result in a 3dB level increase. The best results with respect to evening out bass response by way of minimizing/cancelling room modes do however require positioning the subs at opposite mid-wall positions which are at the minimum position for some modes and partially cancel others because of being located where those modes are opposite in phase. Diagonally opposite corners result in partial cancellation of both width and length modes.
As Alan has pointed out, stereo reproduction doesn't reproduce the actual recording as realistically as well-implemented surround processing does. The majority of the sound at a concert reaches us as reflections from directions other than the front. When the microphones pick this up there's nothing else to do in 2-channel source material(it isn't a matter of "intent") but mix it into the front channels. Analysing the phase differences that the mixed-in surround content exhibits, extracting it from the front and sending it to the surrounds where it belongs is more realistic, with the improvement varying depending on how much ambient content is present in each recording.
