Nikon D6 Reference Manual page 786

Delete Images from Both Slots
Choose whether deleting a copy of a
picture recorded to both memory cards
with either [Backup] or a dual-format
option ([RAW Slot 1 - JPEG Slot 2] or
[JPEG Slot 1 - JPEG Slot 2]) selected for
[Role played by card in Slot 2] in the
photo shooting menu also deletes the remaining copy.
Option
[Yes
(confirmation
required)]
[Yes]
[No]
734 D The Playback Menu: Managing Images
Before deleting one copy, you will be prompted to
choose whether to delete the other. To choose the
option highlighted by default, press 2 .
["Same images on w and x "] is always selected;
selecting [Yes] in the confirmation dialog deletes both
copies.
The confirmation dialog displayed is the same as that for
pictures for which no second copy exists. Deleting the
current picture does not delete the copy.
G button U D playback menu
Description