Two Subwoofers... Comb Effect

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!

I should clarify that the punchy feeling of coherent bass is a kind of compression - it is not something so much as you hear in the sense of a note but more like something you feel. I find if I place the subwoofer too far away from the mains (for example the middle of the room or behind the listener then the notes are still audible but the "attack" is less...I suspect the transient response just becomes wrong...the hit from the beater on the drum head arrives later than the bass by however many feet you are closer to the sub than the speaker (6 feet = 6 millisecs and I think we can begin to perceive this even with bass frequencies.)

Hope this clarifies - I certainly agree that for the sound of the note itself it does not seem to matter where it is placed - so getting an even response makes sense.

If you have a DSP then you can set the distance (sub to listener) and have the processor delay the output to the sub to maintain phase. However this may not work for several listeners seated at different distances to sub and speakers...