Of all the things that can go wrong, even on the most important shoot, the best prepared and never to be repeated shoot, card failure is way down the list.
Dual card slots just play to our marketing-induced paranoia. I don't have dual card slots, and like some other posters on here, if I did have I probably wouldn't use them both. Never had a card failure, but if I did, I'd notice it immediately (almost

) and either swap the card or grab the back-up camera.
Sure, dual card slots are a nice idea, but not a deal-breaker. Nikon will have done their homework here - they have plenty of experience of XQD hate and it doesn't seem to have done them any harm.
ps As I speculated earlier, my guess is that XQD (Sony's baby) was part of the sensor deal with Sony.