When two memory cards are inserted in the camera, you can choose one as the primary card using the Primary slot selection item in the photo shooting menu (0 Primary Slot Selection).
Select XQD card slot to designate the card in the XQD card slot as the primary card, SD card slot to choose the SD card.
The roles played by the primary and secondary cards can be chosen using the Secondary slot function option in the photo shooting menu (0 Secondary Slot Function). Choose from:
Overflow (the secondary card is used only when the primary card is full),
Backup (each picture is recorded to both the primary and secondary card), and
RAW primary - JPEG secondary (as for Backup, except that the NEF/RAW copies of photos shot at settings of NEF/RAW + JPEG are recorded only to the primary card and the JPEG copies only to the secondary card).