2x2x12inch or 2x15inch subwoofers

for what?

I am a fan of a stereo pair of subs, located near the mains, front firing, no ports.

bass imaging is created by both the fundamental's volume (as any imaging is), and by the overtones even if the fundamental is mono

i.e. 36hz 72/144/288 ....

always best to use pre-out to sub, back to main in from sub: that gives the amp and the mains both a break from trying to create low bass

I just re-discovered this track: Mila Drumke, bass solo and vocal

https://www.youtube.com/watch?v=ukrNpHCDSjc

I don't know about streaming, but the bass on the CD is awesome!

Home theater is what this is for.  My 2 stereo subs are set up.  I just want everyone's opinion on whether a sub with 2 12" drivers will output more low end extension than 1 15" driver.

The surface area of the two is going to be close.  Go by the -3 dB rating and see if it makes much of a difference.

The hardest thing with subs is to integrate them into the room and with the speakers.  I would investigate exactly how you would to this with each of your choices. 

A great room correction sub >> one you have to set up manually.

There is no benefit to going smaller either.  The idea that small speakers are faster is a myth that needs to die.

"... The idea that small speakers are faster is a myth that needs to die.."

A Velodyne rep once asked me; 12 inch or 18 inch, which speaker gets there quicker?...I said; They both get there at the same time or they are in distortion.   

Four 10" will be better than any of those. Deeper flatter faster more powerful more articulate. Not even close. Do a search for Swarm, Distributed Bass Array, DBA, etc. The Audiokinesis Swarm Subwoofer system just won an award for best component. See the recent thread and all of us who know just how important it is to have MORE subs and not bigger subs.

For home theater, get as many and as big and powerful of subs as you can afford and have room for, assuming you're not living in an apartment. As far as how deep they'll go, look at the specs. I love and own Rythmik subs, but for sheer output, I'd take a look at power sound audio also. You'll get better information on a home theater forum than you will here. 

My room is 17x24x9.   https://systems.audiogon.com/systems/8367 The focus is music but we watch a lot of movies. The other night we were watching Oblivion and there was a point where they shot Tom Cruise and he fell and there was this bwoommmmm and I don't know how low or what frequency all I know is I felt the earth move. The whole room was shaking. And this is with Podiums, and Pods under all the subs. Absolutely phenomenal bass. Super powerful, incredibly clean, almost like you could see the pressure waves with your eyes. Honestly had no idea the damn thing could even do that!   

I just do not see any dichotomy between music and home theater systems. I do see people following HT dictates and that never turns out well. Follow the very best high end sound solutions, get a DBA, you will see.

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.

Docknow

15 feet...761/15=50ish hz.  That is my room mode that is the loudest and it is +15db, or should I say before my room treatments.  That room mode is closer to +3db.  I use Room Correction to correct the rest.  It goes much deeper than 50hz in this room.

@golfnutz I must be reading too much REL propaganda! I would suggest that all things being equal, the smaller cone is faster but in reality, probably not. I stand corrected.