Well, if flash-to-pass works, but high beams themselves don't (shown by having to hold the switch back), the high beam filaments are probably good. Since the exact same wiring and filaments are used for both.

I'd not worry about them both being burned out until we had high beams all the time. Also, I'd check for voltage out of the switch for low beams if that's the case ... may be easier to check at the headlights, but still, that's what I'd check.

RwP