I beleive the Gannet had two engines (actuall one Double Mamba IIRC) each driving its own prop. I think they used to shut down one engine in the cruise and feather that prop - I'm sure I've seen film of one doing a flypast with the back prop feathered but very slowly turning probably due to the airflow off the front prop.
MOF - I think all twin rotor helos (other than where the rotors are concentric) are interconnected, including the Osprey. This not only ensures that failure of one engine isn't catastrophic, but also in many cses the rotors intermesh, so have to be geared together mechanically.
SSD