Abstract: A twin engine helicopter has an accessory drive system that will operate with only one engine while the other engine is off. The system also allows starting of the second engine without turning the first engine off. An input clutch for the first engine will rotate an inner drive member when it is being driven by the first engine. An accessory drive member mounts to the inner drive member. A collector gear mounts in engagement with the blade drive train and has a drive member. A movable clutch shaft has one end that engages the inner drive member of the input clutch and another end which will engage the drive member of the collector gear. An actuator will move the clutch shaft out of engagement with the drive member of the collector gear to disengage the drive engine from the blade drive train. A synchronizing clutch mounts in engagement with the clutch shaft for causing the clutch shaft to rotate with the collector gear if the second engine is started and operated at a speed faster than the first engine.