Just as a follow up as I don't do much night riding, does the ground return from the headlamps handle the current for a reasonable time. I read an article of another model bike, I think an early K series that there was only one ground return and two lamps was too much for it when someone tried something like this. I don't have a wire diagram that shows the return wiring set up.
Looking at this electrically there is no additional load outside of the added diode. All this diode is doing is triggering a relay which is very little power draw through the diode. The relay is the factory designed relay and it is what holds the "load" for the circuit/lights both positive and negative. So additional ground load in comparison to the factory design would be zero, I / we didn't add a ground or additional load to a ground.
Do not forget the bikes were designed to operate this way outside of the US so we are not increasing the duty cycle of the circuits beyond designed. That is the bonus of this mod using the relays is you are not asking one circuit to energize both lights. Just to hold one additional relay closed.
I do not know the circuits/bike you are referring to. My guess would be that it was never designed to hold both lights on at all times like our bike was designed to do from the factory.
-tim
