nikon – Missing one image in a sequence with a dual card setup

Shooting with a Nikon D810 with two cards:

  • 64GB compact flash (Sandisk Extreme)
  • 32GB SD card (Sandisk Extreme Pro)

The camera is set set to write to both in Backup mode.
I photograph criminal scenes and it’s crucial these are captured securely and in sequence.

After photographing a scene, and in review of the cards I found two issues:

  • The first two images were on the compact flash card but missing from the SD card.
  • A random image in the sequence (image number 34 in a sequence of 60 images) was missing from the compact flash card but on the SD card.

No errors appeared at the time of shooting and cards were formatted in camera prior to shooting.

What can cause this to happen? And how do I assure this doesn’t happen again in the future?