Two Subwoofers... Comb Effect

Halcro, Todd Welti et al published a couple of thoroughly scientific (nothing "quasi" about them) papers on the same subject in the Journal of the Audio Engineering Society a few years ago. What you see at the link above is the version he wrote for non-engineers.

Another very scientific proponent of multiple subwoofers is Earl Geddes, though he hasn't published any peer-reviewed papers on the subject. His approach differs from that described in the Welti papers somewhat; Earl advocates asymmetrical placement. Here is a brief study he did comparing four-corners placement with asymmetrical placement of four subs:

http://www.gedlee.com/downloads/sub%20study%20.pdf

I use a simplified quasi-Geddes placement strategy; I do not elevate one of the subs as he recommends.

Duke

I was logged in on the account of a physically challenged friend (with his permission) when I posted above.

Duke

The 'study' room was 2x as long as high. and 'medium' damped, whatever that is. Was it meant to be 'average'?
All you can say is that IN THIS CASE...the results were as noted. I'd love to see this repeated in my room with 8 walls, 2 of which are at 45's, and an asymmetrical vaulted ceiling.
In the test room, the 1st plots 100hz (aprox) peak can be accounted for since the height of 3.5m is about 95hz at 1100ft/sec.
What I take away from this is that a listening area is not just floor space, but a 3d problem, something I knew before.

I think it's an advantage to place the subs as close to the mains as practical

That has been my experience too. I fear one can get caught up trying to even out the room modal response and lose "transient" response if you move the subs too far from the mains. In a sense, time coherent bass with the rest of the response may be as important as getting rid of the room modes - in practice I use a bit of give and take....I move the sub out into the room a bit in front of the mains but not so far as I feel that transient response suffers. This leaves me with perhaps less than optimal "room modes" but I am streets ahead of the situation with sub in the corner and still get reasonable transients.

FWIW: If you have freestanding speakers then you may not notice this effect - already free standing speakers are sending bass in an omnidirectional pattern - so you lose bass impact from quarter wave rear wall cancellation from the get go ( a set of nulls in the bass across the entire length of the room).

I ditto the experience with sub(s) placed up near the front of the room, with the mains. Either multiple subs in one location, perhaps just off center on the front wall - near the center - , or similar, or subs near the mains, perhaps in stereo, maybe mono, depending.
It's almost always easier to get a better blend, even from multiple seating positions, and still maintain excellent cohesion from the subs to the mains, from the listing positions. It also eliminates any potential for hearing "voices" from your subs, overlap problems, and just plain integrates easier in my opinion.
While I think, carefully done and considered, you can build an excellent system around subs placed around the room, I think the benefits of doing subs up near the mains offers better integration and benefits - especially when you consider today's DSP and EQ technology applied!
Also, I don't personally like to see subwoofer sitting around the room. The gear is an "eye-soar" as it is, mostly. I'd rather be able to have the option to hide the mains and subs behind some facade if I so chose, in a custom set up. And this is all easily done with all the speakers up front, and side/rears hidden, somewhat. But, that's me.
Still, I can't argue with the excellent results I've had over the years with either one, or all of the main/front subs (usually rears crossed over to these subs also) up front. Clients have been ecstatic to!
Still, in each application, you likely will need to "tinker", and adjust for maximum results, and keep a balance.
Bottom line though, is the results is all that matters