Another contribution. I lived with a single sub (powered Genesis Servo 12) and do, or did, hear some lop-sidedness. I use Outlaw ICBM Base Management (an active x-over unit) to filter the lowest HZ to the sub, and away from the low-mid drivers of the main speaker pair.
A second sub (powered REL Q 200e) was brought in by another audio friend and connected at the stereo subwoofer outlet of the ICBM. Mode switch set to stereo, and the blend (combine) to off, and spl level of each sub set as equally as possible.
As I roamed the room, the bass was coming from the left arena and bass was coming from the right arena, so far good. Sitting in the "sweet spot" or SPOT about center and each sub pointing to about the SPOT and spaced fourteen feet, I quickly realized that the bass was centered and not lop-sided. Very noticeable, very blended, very balanced, very different from only one sub. In a nutshell, it is bigger-bolder-better. Important to this is the sound levels of each sub is adjustable. Also, I do not have access to and use of corners like some of you have. My sound room is also owned and used for sitting by my wonderful wife.
I bought a second sub (used Genesis) after one week of listening, yet would have after the first thirty minutes. Never any regrets, extremely happy. Difference to me was between 5% and 8% improvement.
Sound level of entire system with two subs is simply controlled by me at the preamp (First Sound Reference, a dual mono design). ...Lower frequencies may be less directional, yet in a larger room size, you have to fully fill a room with sound. One sub just doesn't quite cut it, as the room size enlarges. My space is 18 x 22 x 16 (lwh).
Thanks all for reading....