allowing pilots to fly with different generations of NVG in the same cockpit (who can see what the other can't?)
I flew RAF helicopters when the SOP was that the PNF wasn't even allowed NVG, although the PF and crewman were. This included confined area landings. I was the PNF and was expected to take control and save the day if the goggles snowflaked out. They did, just inside the top of the CA, which was in a very large German forest, and I had to!
Edit: Actually, it was PNG, the RAF hadn't got NVG in those days..