I agree with 16 Blades in as much as Herc Co-pilots are often treated as U/T Pilots, and have been for years, rather than U/T Captains. All Co-pilots are potential Captains and some Captains should do more to bring them on.
I disagree that Co-pilots need to fly in LHS. Where is the advantage. Training as Captains, in all aspects of captaincy, can be done when they are in the RHS.
Spanish Air Force had an interesting training method seen during GW1 - 1 Captain, 2 Co-pilots, no Nav. Each day they moved round one seat so on day 2, 2 Co-pilot in front, Captain at Nav station.
Canadian Air Force used to have a policy of junior and senior Co-pilots. On becoming a senior, short course then allowed to fly LHS with some restrictions, may also have been allowed MCT with two Co-pilots but memory is faiding.
The disadvantage, and the only one I can see, in Co-pilots flying LHS is during an abort. They can be trained in the sim to a competent standard but the responsibility for the aircraft lies with the Captain. He, as a training captain, has the right to brief that he will take control during an abort from the RHS. Been there, done it!!!!!! By the time you realise that it is time to take control it can get very very hairy.