I've received a question about the Canon EOS 5D Mark III DSLR's write speed using SDHC and CF Cards simultaneously. I thought I would share the answer with you.
With a Lexar Professional 32GB 1000x UDMA-7 CompactFlash Memory Card installed alone, the 5D III will deliver a burst of 33 RAW images (photographing a dim scene).
The 5D III's max SDHC/SDXC write speed is slower than its CF max write speed - and the UHS-I standard is not supported. With a Lexar Professional 32GB 133x SDHC Memory Card installed alone, the 5D III will deliver a burst of 16 RAW images (photographing the same dim scene).
Put both cards in the camera at the same time and the 5D III's write speed will be the speed of the card being written to. Writing to the 1000x CF card will result in a 33 RAW frame burst and the 133x SDHC card will take 16 RAW frames.
By default, the 5D III will write to the last card slot used. You can change this card slot designation in the first tool menu tab (Record func+card/folder sel). The card slot numbers are indicated on the memory card door. CF is slot 1, SDHC is slot 2.