Wasup my main man, Steve?
I actually found the "natural breathing" of a 2x12 to be the biggest benefit. It just seems that a 1x12 open-back is more of a sonic "spotlight", and a 2x12 open-back is more of a sonic "floodlight". The 2x12 offers wider attack, and a broader "sweet spot" without the spikey center. Hmmm, I do not know if you remember the oversize tennis racket craze in the early 1980's...but the wider "sweetspot" rendered the old rackets obsolete.
Do I feel that 1x12's are obsolete? NEVER! I actually sold my Carvin 4x12 after got a 1x12 Fender cab loaded with 1, 200 Watt EVM 12L speaker years ago. I used my ears. The quality of the EVM speaker said YESSS! This was over a decade before the internet so, it really was the Dark Ages. If I had happened to stumble onto a 2x12 at the time...I may have taken it. Speaker quality does play a major part of this whole dicussion.
Your added indepedent cabinet sounds great for sound dispersion AND cabinet placement flexibility.
Does a 4x12 make a better sonic "floodlight" than a 2x12? Nooo Wayy! Usually 4x12 cabs are closed back and are too often like Lighthouse "searchlights"...all projection and to little open-ness in any room smaller than an empty two car garage (too much like trying to swing a shotgun around in a closet!).