Nikon D6 Reference Manual page 482

A
Deleting Copies
If the picture selected for playback when the O ( Q ) button is pressed was
recorded with two memory cards inserted and an option other than
[Overflow] selected for [Role played by card in Slot 2] in the photo
shooting menu, you will be given the option of deleting both copies or
only the copy on the card in the current slot ( 0 311).
430
Deleting Pictures