Sounds spooky, huh? As long as the 6v POS GND and the 12v NEG GND circuits EACH remain as a closed loop unto themselves, the frame (for instance) can be used as a COMMON ground.
How many among us has questioned about safely using a 12v battery to jump-start our 6v POS ground cars? The logical answer has always been to first make sure that ALL 6v accessories are turned-off (such as a 6v POS gnd. radio), so that 12v and "unfriendly" grounding CANNOT be realized by the radio, EVEN THOUGH the radio remains grounded to the chassis. Second step is to turn-on the normal 6v POS grounded ignition switch (to power the coil). Third step........connect 12v POS jumper cable to big starter lug. Then, connect 12v NEG jumper cable to starter case, or engine block, or chassis ground. The starter will crank until you remove either of the jumper cables. Most-importantly, you have just mixed 6v POSitively-grounded....with 12v NEGatively-grounded (using the SAME, common ground), and all is still well. DD