Assuming you’re going for primarily music, one of the smallest 12" subs are the SVS SB1000 Pros. I have these in a dual sub setup with my Wharfedale Lintons. Lintons at front wall and my subs are on opposing side walls. Did some sub crawling, but primarily went by my REW measurements and ultimately my ears. I often have to go to the subs to see if they’re on, but you can definitely tell when they’re off. Guess I’ve blended them well enough thru measurements and then trial/error. My room goes down to 28hz and I have two corner bass traps. Other than that it’s a carpeted room with a thick curtain over my glass windows. Sounds fantastic and the SVS’ have another great feature - bluetooth app so you can adjust the settings from your listening position.
My final comment is I didn't go dual subs for extra bass, I have enough, but the dual subs handle the nulls mentioned by others, but for me just give a more immersive "presence" with the soundstage, depth, etc.