I have been working on cars a long time. Over the years I have seen both front headlight fail the the same time many times. I 'm not sure what causes this. However, I think a voltage spike occurs when one light fails. The collapsing -15 - 20 amps fieled could flow back to the other lights since they are on the same circuit. This failure also occurred on older cars with independent voltage regulators. However, I have never since this on a generator car. Bob Brandys