Canon EOS-1DX Instruction Manual page 237

k Shooting Movies
AEB cannot be used.
Even if an external Speedlite is used, it will not fire.
Continuous still photo shooting is possible during movie shooting.
However, the captured images will not be displayed on the screen.
Depending on the still photo's image-recording quality, number of shots
during continuous shooting, card performance, etc., the movie shooting
may stop automatically.
If [z5: Movie shoot. btn] is set to [V/o], still photo shooting is not
possible.
If you want to shoot still photos continuously during movie shooting,
using a high-speed card is recommended. Setting a smaller image size
for still photos and shooting fewer continuous still photos are also
recommended.
You can shoot still photos in all drive modes.
The self-timer can be used before you start shooting a movie. If used
during movie shooting, the self-timer will switch to single shooting.
237