When my father's Stirling was shot down over Belgium all eight on board were killed because he had a second dickey with him otherwise it would have been seven. Feb 1943
In Jan 1942 my mother's brother had a second Pilot with him so I guess between those dates the RAF went to just one Pilot. No doubt due to losses? He survived coming down close to Hamburg in the sea and was saved by a Doctor fresh from the Russian front who was used to low temperatures .