There were trials of continuous double-crew ops on both Belfast and VC10. The VC10 trial was in July 1969 and I suspect that the Belfast one was also around that time. Slip crew positioning for the Westabout ATF Exercise Hosanna in January 1970 was accomplished by a double-crewed VC10. I stand to be corrected on this, but I'm not aware of the technique being used after that. (And whilst the VC10 did not have bunks like the Belfast, the Squadron F540 report on the VC10 trial suggested that the medics ended-up more knackered than the crews!)