the headlight switch turns power to the headlights on/off, it does not select between hi/lo. if your highs are on, the headlight switch should turn them off.
so if you can indeed switch the Highs off, the hi/lo switch seems to work ok. next can you confirm that you are indeed getting 12 volts to the low beam wire of the headlight socket? I'd hate for all of this to be caused by both low filaments being burned out by coincidence.