I use 2 cards, One 32 GB or so in slot 1, and one 8GB in the bottom slot. Using the top for NEF and the bottom for JPG, I find that they fill up at about the same time. That's the way it worked on my D7000, where I got into the habit of doing this. My D600, where the files are much larger, works out about the same. I have had no incidents of JPGs mixed in with NEFs, or vice versa except when I forgot to put one of the cards in, and I've been doing this for 2 years. Just a suggestion, but separating the two file types on different cards may help you keep the separation cleaner, and do what you want.