How does adding a second subwoofer improve your systems SQ?

simple answer

1.  2 subs load the room better, in many applications

2.  bass is supposedly non directional, but in reality, subs have crossovers that 'leak' into higher, directional frequencies... stereo subs put these in L&R space where they belong (moreso)

markj941 OP

If members here that are pro one sub, and running it off a HT sub output.
Forget that single HT preamp sub output, get two subs run both subs from the main L & R preamp outputs 
You will find if you have two subs and in the HT programming "set the mains for large" with "no sub" picked, then cross the 2 subs in at where your mains fall off, get the phasing and level right ect.
Your in for a treat from what can come from active proper stereo setup bass bins.
Your mid sized floor standers or large book shelves will have the scale of sound that you only get from big floor standers like Wilson Alexia Max Alex ect.

Cheers George  

@erik_squires , you already posted a link to your blog page where you wrote: " I am no longer a fan of this idea [distributed bass array/muliple subs] due to the fan boys and how cultish they have become. "

And now you post: "... I often challenge the DBA cultists..."

This is starting to look like trolling, Erik. 

Maybe you can advocate for your preferred solution without name-calling? 

Duke

Duke,
You already know where we stand.  I won't be editorialized by you.
Erik

" I won't be editorialized by you." 

I suppose we're both free to call it like we see it. 

Duke