I'm running at 50% success rate here and I need a bit of help. I connected two Flipsky 4.2s via CanBus which ended in ESC death. They did have two different batteries and I'm thinking the voltage difference might have crossed the two CanBus wires. I tried again with two Flipsky 75100s and it worked with no issue, though the voltage difference may have been less. Do I need galvanic isolators across the Can Bus to avoid any issues when using separate power sources? Should I do the same even if using the same battery for all motors?
You need to make sure that GND between the two power inputs is always connected. You switch the units via the positive battery connection only. The 4.xx based design will probably not have a 60V rated CAN transceiver and will not like it if you plug them in independently. So make sure Batt GND is bridged at all times. The original VESC branded controllers/devices have 60V rated CAN chips and some also feature isolated CAN. Third party may feature lesser rated components.