If you’re running amp to sub to bookshelf and high passing the bookshelf speakers then it seems to be essentially using a sub for each channel and if you’re running subs that high (80+) I’d situate them very close to the speakers to get ’full-range’ speakers.
Otherwise, if not high-passing the mains, I might consider two asymmetric locations for the two subs (not necessarily by them) for better low bass integration in the room and low pass them lower than normal. Depends on the small speakers I’d say.
I moved from an REL B1 to three active SVS subs and added another sub recently for four total, set up asymetrically in my main room. It works superbly for really low end bass, and stays out of the way when not needed. I'm a +1 on D.B.A. setups.