How does adding a second subwoofer improve your systems SQ?

Let me give you a practical example-

In my 2-channel setup I basically have only two small areas where my left and right subwoofers will fit- so positional flexibility is very limited.

I recently measured the response of each sub during my integration process.  The left sub had a dropoff below 30 Hz and small peaks above 60 Hz.  The right sub was flat down to 20Hz but lacking some impact.  
When both playing together the response is smooth and flat at my listening spot, sounds exciting, deep, dynamic and smooth.  
Having two subs is a great advantage to achieving a balanced sound if you have limited room to position them.     

I get exactly that effect using only one sub in my bedroom system.

You are getting a just sample of what’s going on down there, you are drawing conclusions about the Roger Waters track with regards to hearing it with two L and R subs, it’s obvious. Because if you did you wouldn’t be saying what you just said.

I done the a/b with 1 mono vs 2 in stereo. And my subs are at 50hz 4th order, and the difference is huge.
Anyone who mono’s their bass below 100hz is ruining many albums that have been recorded in stereo down there.
  
It may not matter to the vinyl crew so much because they’re lucky to get 10db of channel separation down that low from their phono cartridges, so they are basically listening to mono anyway.

Good article on speakers  by Floyd Toole, close to the end there’s a section how to get great sound from a single subwoofer. As long as you’re only interested in one listening position.
https://www.harman.com/Documents/AudioScience_0.pdf

Avanti,
Having two subs is a great advantage to achieving a balanced sound if you have limited room to position them.    

This is what I was waiting for. Unless you have a dedicated audio room, I don't, there are definite limitations to most all rooms. My system is in my living room. I simply can't put subwoofers where they might sound best. I'm confined to placing my sub next to one of my main speakers. Avanti, so your thesis is, buy a second sub and place next to the other speaker and enjoy the music. 

I am a huge fan of Floyd Toole, but in general he seems to not consider affordable bass traps as solutions. He kind of alludes to this:

Short of hiring an acoustical consultant, and being willing to rearrange the furniture and possibly rebuild the walls, what can be done?

So his entire solution space is outside of this possibility, which may be a choice based on who they expect their customers to be.

Other, more modern acousticians have written with the presumption that bass traps are an option and they don’t throw up their hands quite as easily to the problem.

Once we allow for bass traps, things get much better for subs and EQs, and it is from this perspective that I often challenge the DBA cultists. A well integrated single sub, in a well treated room, including bass traps performs really amazingly well.  Two subs is better.

Best,
E