Canon’s newest 1DX Mark III firmware update allows the camera to record video to both CFexpress cards. Also added is C-Log 3, yet another video codec attempting to squeeze more dynamic range out of video files and keep post processing professionals employed.
While the R5 also has dual cards, they are not both CFexpress Type B cards, with the R5 sporting an SD card second slot. This might be why the firmware goddess failed to bestow similar gifts to users of that body.
The R6, though, does get the C-Log 3 update with its own update.