There has to be a common connection between the purple wires somewhere. As I can recall it only goes from the ignition switch run position, gauges, coil and on 3 wire alternators I believe it also goes there. Have the alternator/s been replaced? This one had me scratching my head all night. Interesting.

I know all the gauges have and I think it is in the volt meter wires behind dash I know it has me going crazy figured if wires were jumped it would not matter if u shut off oneside or the other battery switch off but it does

It's has to be something dumb

yl red strip is ignition start relay......purple is +12 swithed through the key to the coil....

When I lost bothink engines within a 150 feet of each other I was idling lost spark in both only thing I did find is one battery was bad