Properly grounding 2 MCUs using I2C
I would ask the following question: is your low power linear supply transformer-isolated? (They usually are.) That is, does it use a transformer to convert mains voltage to a low AC voltage, then convert to DC and regulate? If so, and if you can float the low voltage lines, simply tie your two DC grounds together with a convenient gauge wire. The step-down transformer will isolate the output from the input. 

If you do this, there remains the possibility that the low-power chassis may still be connected to a different ground line than the high-power chassis, with the potential for hazard. And it will be necessary to look at the linear supply schematic to see if you can actually disconnect supply common from ground. It will also be a truly excellent idea to verify that both mains connectors are properly wired. If not, the two circuit grounds may actually have mains voltage across them.