5D Mark IV - Dual Slots for 4k?

If your SD card and your CF card are both able to write fast enough, can you record the same footage to both cards?

I read on a few sites that it can't write to SD at all, but on others that it can if the card is good enough. However, I found no information about my question: can it write to both, at the same time?

Video can only be written to one card.

Assuming you have fast cards in both slots, set camera to record 4k to the CF card. You'll get a bit better performance. You can record to either card, but as previously stated only one card at a time for recording video.

The 5D mark IV only supports UHS-i SD cards. SO anything 100MBS or slower pretty much. If you want to shoot 4K 24/30, it needs the CF...