Heres how we do it in the C-17 world.
Everyone is left and right seat qualified from the start.
An augmented crew is required for duty operations over 16 hrs, max duty day is 24 hours (there are a couple caveats in minimum leg length too)
Augmented crew (pilot wise) consists of minimum 1 Aircraft Commander (captain) and 2 First Pilots (co-pilots)
Now you can staff the crew with 3 Aircraft Commanders if you want and you can have any combination of pilots in the seat for take off and landing regardless of who signed for the jet (A code in AF parlance)
A lesser experienced First Pilot or FP can only sit in the right seat for take off and landing if a Aircraft Commander or Mission Pilot (MP) occupies the left seat. If there is an Instructor Aircraft Commander or IP in the other seat, that inexperienced guy can take off and land from the left or right seat.
A more experienced FP can occupy the left or right seat regardless if there is an MP in the left or right seat.
An MP can occupy either seat for take off and landing.
Clear yet? We have a lot of flexibilty in who operates where, but the duty days can be long and only have 3 pilots, whereas a 121 airline would be required to staff the jet with 4 pilots.