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.

HSU Research recommends 'near field' sub placement. When I was up there (OC, SoCal) I sat in there listening/audition area and sat virtually next to the sub(s) under test. Still difficult to localize and the 'blend' was good.
If I could work it out, I'd TRY my sub in back of the sofa, concealed (sort of) under the sofa back table. However, I got lucky and near corner placement (3 feet out) and against a long wall made it happen.
I have the advantage, by sheer accident, of having a room with 8 or so walls, 3 45degree corners and an off-center asymmetric ceiling of 12' max and 8' at one side. This room seems to break up or not be conducive to the formation of 'bad' standing waves....(so called peaks or suckouts).

Treatments ARE in the offing, but the idea would be to kill first reflection points from my Maggies and deaden the back of the room....the back wall is a reflection monster, too.

MAAB, please be careful, and Shad, this is meant with all due respect, of any and MOST prescriptive advice. Your listening area is a unique problem and Opportunity.
Experiment around. Even if you can't leave something there due to WAF, try it. Send everyone OUT for the day. Invest in some movie tickets for 'em all. Get RID of them so you can move stuff around and experiment. That's how I discovered, many years ago, that the one hard/fast rule of Magnepans.....IE, tweeters OUT doesn't apply in ALL instances. You will learn stuff nobody could tell you about your room / system / ears / interactions.
It'll all be good.

One key is to keep the x-over as low as practical. This will make sure you can't localize the bass. My Maggies, 1.6s are x'd-over at about 40-45hz. Seamless and invisible.
Phase, however, is another matter and does make a difference.
I wish I had a fully variable 0->180 control rather than the 0 OR 180 I now have. Small point, but don't forget that sound travels at about 14"/millisecond, so a small movement of the sub can produce phase changes, too.

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.

That's why I x-over as low as possible...40->45hz. And while my Sub IS fairly close to the LH speaker, it is still pretty seamless. Don't forget that recordings. ..with the possible exception of the Telarc recording of Wellingtons Victory are MONO below about 80hz.
The Telarc disk has very localized CanonShots which are a real system test.
SMALL point...using 1ms/ft is probably OK, but sound is actually a little quicker....maybe 14"/ms....like a said, a very small point......