Canon EOS-1D X Mark II DSLR Camera (Body Only) - Manual (Page 515)

515
Troubleshooting Guide
For High Frame Rate movie shooting, if [ Free run ] is set for [ Count
up ] under [ z 5: Time code ] (p.326), the time code will not be
appended. Also, if there is HDMI output, the time code will not be
appended to the HDMI video output (p.328).
For High Frame Rate movie shooting, it will count up by 4 sec. for
every sec. in real time (p.318).
Still photos cannot be taken during movie shooting. To take still photos,
stop the movie shooting and then perform viewfinder shooting or Live
View shooting.
The time code is not appended.
The time code’s count is faster.
I cannot take still photos during movie shooting.