There was no clutch system in the venerable old Sea Pig.
As described above you would start the number one engine in 'Accesory Drive' which was a selectable freewheel which enabled the gearbox through shaft to drive the accessories. This allowed one engine to be started to power up the generators and the hydraulic systems thus allowing blade fold/spread. Normally the number one would be run at 104% to prevent chattering when the main rotors were engaged.
Once the aircraft was ready for rotor engagement the number two engine was started against the rotor brake to prevent rotor turning as the free power turbine is directly connected to the input drive shaft. Pre torquing the number two before releasing the rotor brake enabled a smooth acceleration of the rotors. Once the rotors were up to speed the number one engine would be brought back to ground idle to enable the freewheel to be switched over from the through shaft to the main input shaft. Then both engines are brought up to speed to match torques and drive the main rotor.
There are no clutch systems within this drive train. There are multiple freewheels but these are only used to allow the rotor system to rotate in the event of an input bearing seizure. Also the freewheel within the acc drive system allows the drive to be split between the through shaft and the input shaft. This system can ONLY be switched when there is no load on it. Otherwise you get the famous '1 leads 2 Oooops!' Sea King clunk!
W2P