Nikon D6 Reference Manual page 362

A
[Backup], [RAW Slot 1 - JPEG Slot 2], and [JPEG Slot 1 - JPEG
Slot 2]
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The viewfinder and top control panel show the number of exposures
remaining on the card with the least amount of space available.
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The shutter release is disabled when either card is full.
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Viewing Dual-Format Photos
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Use the [Dual-format recording PB slot] item in the playback menu to
choose the slot from which dual-format photos are played back.
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When viewing dual-format photos, you can view the other copy using
[Jump to copy on other card] in the i menu.
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Recording Movies
When two memory cards are inserted in the camera, the slot used to
record movies can be selected using the [Destination] item in the movie
shooting menu.
310
Using Two Memory Cards