I imagine it then became feasible for Transport Command to insist on pilots doing co-pilot tours before graduating across to the other seat.
They started training Co-pilots on Hastings and Beverleys after the introduction of the Britannia. Second pilots were not qualified on type. For many years there was no way to progress. Second pilot hours were worth nothing. We had to do at least 50 P1 hours a year in a Chipmunk or Anson just to qualify for flyng pay. My two week Hastings course covered climatology, load sheets and how to operate the undercarriage and flaps - utterly soul destroying for a wannabe Hunter pilot, who's course was cancelled courtesy of Duncan Sandys.