Some Nikon D-SLRs have two card slots… but why?
It gives you wider compatibility with an existing collection of memory cards. For example, the D800 takes both CF and SD cards.
The Nikon D4s is a special case, because one slot takes conventional CF cards while the second uses the new, super-fast XQD format.
Some cameras, like the Nikon D610, have two identical SD card slots. The second card can act as extra storage (overflow) when the first is full.
The second card can also be used to keep a backup of your images as you shoot – if one card fails, your images are on the second.
And if you shoot RAW+JPEG files simultaneously, you can save each file type to a different card.