Experiment with different locations. Subwoofer effectiveness is very dependent subwoofer and listener placement.
My first thought was “Why in the world are they sitting where they are?”
Read a few article on subwoofer placement and see if you can’t find better spots.
Once ready to try, put both Subs on extension cords and try different locations to see what sounds best. Might even just use one to start with to simplify finding locations and then add in interactions after you have found sweet spots.
Trueplay if possible, if not check the phase on each to see which combination sounds best.
I would rethink the soundbar as he front stage and use an Amp with passive front speakers instead. That way you can use a non Sonos sub/subs instead of the Sonos subs.
