I've lit dual filaments for a while... I definitely experienced shorter bulb life, I suspect because of the increased heat. On an earlier vehicle that I bridged similarly, I burnt out the ground, so I doubled-up on my Amigo and ran a parallel heavier-guage second ground. No failure yet.

What I did was study the wiring diagram to identify the two signal leads for the high and low beams (each switches a headlight relay) and then switch them together. This way I can run high, low, or both. I eventually ran a diode inline with the switch so that the current ran through the switch only from high to low... that way I could still use the 'dip headlight' feature and switch back to low beams normally.

Randii