A single engine helicopter with a free power turbine does not require a clutch. But all helicopters require a free-wheel unit to permit autorotation following a turbine seizure
Kieth, I think you are confusing terminology. The free wheel unit on every helicopter I've ever flown, single or twin engine, was provided by a sprag clutch.
Twin engine helicopters usually have a clutch on one engine. This permits that engine to be started first to drive accessories such as hydraulics, with the main rotors stationary. After the second engine has been started and the rotors run up to normal RPM, the clutch is then used to connect the first engine to the rotating rotors. Both engine swil then be driving the rotors.
Each engine will be connected to the main transmission through a separate free-wheel unit. But only the engine that is to be started first needs to have a clutch.
What type of helicopter uses this type of set up? None that I've flown I'm afraid (76, 212, 412).