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.