Lancasters (all heavies I think) had one pilot. I think iirc early in the war Harris / the RAF took the decision to have one pilot simply because two pilots should be flying two 'planes not one.
Obviously for training reasons etc sometimes a second pilot flew in t'other seat (flight engineers seat I guess). I understand from reading several books it was not uncommon for the flight engineer to receive some flying training so in the event of the pilot being incapacitated there was a chance the FE could get the 'plane back. I'm not sure at all how common this was nor how 'successful'.