I love my Mark IV and its video, but can not figure this out.
I assumed the Mark IV would record video to the CF card, assuming it was rated for video and it does, but it also records to the SD card and I can not tell why or when it switches. I found this out when at a wedding I get the message that card 2 was full, I was using it for video only at that time.

When doing photography I write raw to both cards (record to multiple) I do not see a setting to change this for video.

Any thoughts?

I haven't tested it but I also had this question and read in the manual:

Even if the [5 Record func+media/folder sel.] menu option has been
set to [Rec. separately] or [Rec. to multiple] (p.57), a movie cannot be
recorded simultaneously to both the CF card <f> and SD card <g>. If
[Rec. separately] or [Rec. to multiple] has been set, the movie will be
recorded to the card which has been set for [Playback].

How to set for playback I haven't tried.

I also found that and that is exactly how it works. When recording to multiple cards, the card that you choose for playback is the card the video will be recorded to.