Canon EOS-1D X Mark II Manual (en)

You can merge multiple split movie files composing a movie clip and 
save them as a single file. The original movie files are kept as they are 
in the folder they were originally saved.
Merging All the Split Movie Files in the Folder 
and Saving as a Single File
Click the Merge split files button in the main window.
The [Merge Files] dialog box appears.
In the main window, you can also select the [Tools] menu   
[Merge split files...] to display the [Merge Files] dialog box.
Merging Split Movie Files and Saving as a 
Single File
Merge split files button
Click the [ 
 ] button for [Folder for files to be 
merged] and then select the folder in which the 
movie files to be merged are saved.
All the movie files saved in the selected folder will be merged.