Lu,
You are right - the UH-60 pilot cannot turn off both (in fact three) of the HYD systems at once, and the aircraft cannot be flown without one of the systems (at least thats what it was like in the simulator).
Boost off refers to the isolation of the collective and yaw boost servos, not the primary servos. Bit like flying the UH-1H HYD off, but without the cyclic forces.
