Update for those following along.
The wiring shown a few posts previously has one sneaky failure mode. If the path between the main contactor and the second battery should become open for some reason (broken wire, failed terminal, fusible link or diode burns open), the ignition would continue to operate normally...until the battery ran dead. It's not a critical failure; the other ignition would be unaffected. However, the "mag drop" is so small that a pilot might not notice. The only other clues might be a change in EGT and perhaps some slight roughness if LOP at the time.
It needs an voltage monitor, something to draw attention to a loss of alternator voltage on the second ignition. The EFIS already monitors the main bus, so that's covered. My GRT Sport doesn't have an available input for the monitor task, so I went looking for a suitable stand-alone device. This one fit the bill, and it was a whopping $4.59, delivered
The digital display is for setting the upper and lower voltage limits, using the three buttons. It's a "set and forget"; the board is mounted in a sealed box under the panel. If line voltage goes over or under the limits, it closes the relay and sets off a buzzer. I installed a flashing LED under the #2 ign switch, and set the lower limit at 12.8V:
Function is dirt simple. The LED flashes when the ignition switch is moved to ON, prior to engine start, as battery voltage alone is below 12.8. That's the self test. It stops flashing when the alternator comes on line after start. If it flashes in flight, there is something wrong. The buzzer can't be heard in flight, but it's a perfect volume to warn that I've left the ignition on after shutdown, a nice bonus.