Multiple Batteries not in parallel or series, what to do with ESC grounds?

I have 2 batteries onboard my large and awkward quad that are each powering 2 motors. The batteries are neither in parallel or series, they are not connected. It’s the only way the design will work, im aware of the drawbacks of this but we are past that point. My question is what to do with the ground wires of all the ESC lines? Normally i would connect them up to the Pix when using a single power source but I’m a bit at a loss since with 2 separate, or even 4 (one for each motor if it comes to that) batts with what to do on the ground ESC connections to my pixhawk. Anyone ever done anything like this, or more importantly, anyone know what to do? Thanks in advance.

use TRUE opto isolated escs.