Yes I doubt there is an easy answer to your question Hipper,I have seen crew photos with either 1 or 2 Pilots included.
I was under the impression that some crews used the Bomb Aimer in the cockpit for T/O and Landing and also when not req'd to assist with Navigation.
The Flight Eng would perhaps be busy down in the bilges with his man sized controls.
Of course most Bomber Pilots did 1 or 2 'Stooge' trips with an experienced skipper before being declared operational.