Two Subwoofers... Comb Effect

Comb filtering is not an issue at low frequencies because the room response swamps out any lobing due to multiple sources. Because the ear is literally slow to register bass frequencies, it cannot distinguish between the first-arrival sound and the room response.

On the other hand, multiple subwoofers widely spaced is an effective way to smoothe out the in-room response in the modal region. Todd Welti of Harmon International wrote a paper on the subject:

http://www.harman.com/wp/pdf/multsubs.pdf

Duke
dealer/manufacturer

Above link is a very recommended read.
Key appears to be asymmetrical placement.
I believe wavelengths to be too long for such effects.
50hz has over a 20ft wavelength...as long as many rooms.

thank you very much. That is very informative and reliable, so it seems.

Make sure you use symmetry along the room length I find that if you vary
placment too much along the room length then you can indeed run into
problems => bass becomes muddy instead of punchy. - it loses dynamics or
correct phase on transients. I find best placment is close to wall where you
place speakers and about 4 to 6 feet out (not in a corner) and right up against
side wall. This seems to reduce tangential room modes which have nulls
down the middle. Your only big problem then becomes room length modes
which require acoustic treatments at each end and notch filtering in the bass
on the modal peaks....as usual YMMV.

Duke, your room sounded great at RMAF; I especially loved the clean off-axis presentation! Kudos to you for thinking outside the (subwoofer) box!