2x2x12inch or 2x15inch subwoofers

I don't know if this helps or not, but I have a pair of the E15HP2's (4000 cubic room w/ full acoustic treatment).
If I'm watching a movie with my wife I can use both subs (main system volume is lower than watching a movie myself).
If I'm watching a movie alone, and she's upstairs on either the main floor or the 2nd story floor, I have to turn one of them off, and the volume down on the other. It's amazing how much these things can pressurize a room.
I thought about a pair of G22's, but didn't like where the plate amp is situated.
Best of luck going forward whatever your decision is.

Golfnutz, thank you it does help.  I am contemplating that subwoofer.

I just want everyone's opinion on whether a sub with 2 12" drivers will output more low end extension than 1 15" driver

Both specs on their website say 14Hz lowest bass response. Smaller drivers are generally faster.

Believe what you want...

https://www.audioholics.com/loudspeaker-design/10-nonsense-myths

Read myth #9 -  Myth: smaller subwoofers are faster than larger ones. Big subwoofers sound slow and inarticulate.

What is the longest dimension of the room (think in 3D)?  That will let you calculate the lowest bass note the room will give you.