Thanks for sharing your experience!In my experience, the second sub can only make a difference at a single listening position, if you find a position for it that helps smoothing out deficiencies of the other.
These are my subs before and after correction (ignore SPL), but i needed a lot of trial and error to find the best positions.:
View attachment 306485
I don't really have many other options for placement, the subs are currently placed against the wall behind the main loudspeakers, at 1/4 and 3/4 of the room length, were they should work best in theory at least.
I had/have an issue with a null at around 80 Hz which was and is way more pronounced on the left speaker. I was able to fill most of the null by removing (!) the bass-traps behind the mains, the right speaker was very flat from about 30Hz to 100Hz.
It came back when I moved my desk with the mains closer to the wall to improve the low-mids (SBIR), but only after recalibration, probably because the time-alignment was off and MA-1 corrected that.
Somehow I thought that the much deeper null on the left speaker had something to do with the subwoofer being on the right and that a second one would help to fix that.
I'm going to experiment some more and report back with my measurements.