Nikon D6 Reference Manual page 1032

Image Overlay
Image overlay combines existing photographs to create a single
picture that is saved separately from the originals. One NEF
(RAW) image can be overlaid on another using additive
blending ([Add]), or multiple images can be combined using
[Lighten] or [Darken].
Add
Overlay two NEF (RAW) pictures to create a single new JPEG
copy.
1
Select [Image overlay] in the
retouch menu.
Highlight [Image overlay] and press
2 .
980 N The Retouch Menu: Creating Retouched Copies
G button U N retouch menu