Canon EOS 5D Mark III Instruction Manual (en)

 Selecting the Card for Recording and Playback
Images will be recorded to the card selected with [Record/play].
Auto switch card
Same as with the [Standard] setting, but if the card becomes full, 
the camera will automatically switch to the other card to record 
images. When the camera switches to the other card, a new folder 
will be created automatically.
Rec. separately
You can set the image-recording quality for each card (p.121). Each 
image is recorded to both the CF and SD cards at the image-
recording quality you have set. You can freely set the image-
recording quality to 73 and 1, c and 41, etc.
Rec. to multiple
Each image is recorded to both the CF and SD cards simultaneously 
at the same image size. You can also select RAW+JPEG.
If the image-recording quality settings of card 
f and card g are different 
when [Rec. separately] is set, the maximum burst will decrease greatly 
When [Rec. separately] or [Rec. to multiple] is set, the image will be 
recorded under the same file number to both the CF and SD cards. Also, the 
LCD panel will display the number of possible shots of the card having the 
lower number. If one of the cards becomes full, [Card* full] will be displayed 
and shooting will be disabled. If this happens, either replace the card or set 
the recording method to [Standard] or [Auto switch card] and select the 
card with space remaining to continue shooting.