Canon EOS-1D X Mark III EOS-1DX Mark III Advanced User Guide - Page 611

Note When the camera is set to record RAW images to one CFexpress card and JPEGs or HEIFs to the other, specify which images to transfer in the [RAW+JPEG transfer] or [RAW+HEIF transfer] setting. Similarly, specify your transfer preference when RAW+JPEG images or RAW+HEIF images are simultaneously recorded to a single card. Captured images are also stored on the card. When images of the same size are recorded to both cards simultaneously, images recorded to the card selected for [ Record/play] or [ Play] in [ : Record func+card/folder sel.] are given priority for transfer. 611