With my D300 and with my D500, I have always used the second card for overflow. In twelve years shooting underwater, I've never needed a backup card and I typically shoot over a hundred shots per dive on at least 100 dives per year. I've found the overflow much more useful for topside shooting, where I might shoot 1000 high speed shots at a motorcycle race or while whale watching and them cull out 900 of them. Video also makes for full cards, although with a 64 gig XQD, I've yet to come close.
- kirstyA00 likes this