I think before the war all multi engined bombers ( aside of the Hampden) carried 2 pilots. The change back to single pilots was more to do with the expansion of the force and the introduction of the 4 engined heavies/ flight engineers being introduced.
That is true - however, by a quirk of the "rule book", it also applied to the Hampden for a while....the two pilot Hampden rule disappeared before the introduction of the flight engineer. On a similar subject, I have vague memories of introducing the Sentry into RAF Service, with some wag from Group insisting that the Sentry crew carried out regular parachute drills iaw the 11 Group Flying Order Book. There were many rules that didn't specify FJ crew only and the documents had to be amended quite quickly.