This isn't a good idea if, like most of our cars, your high and low beams are produced by the same bulb. You throw a 100% overload on the common ground path of the two filaments, and you risk grenading the bulb with enough force to destroy the headlight reflector and lens. The bulb is pressurized to about 10 atmospheres cold. Internal pressure goes (way) up when it's hot, and if you now come along and throw 200% of the expected heat at it because both filaments are operating at the same time, you're playin' with fire.
If you have a car that produces the low and high beams with separate single-filament bulbs, you can safely do this as long as you make sure all the wiring's up to the task -- feeds and grounds alike.
