I shoot sports, so I do RAW to Card 1, and JPEG to Card 2. This goes for the 1DX and 1D4 as well. Not much sense in sports to wait for the time lapse for both RAW's to go to to their respective cards. Weddings and baby portraits, I do RAW to both, just as backup. If nothing goes wrong with Card 1, I don't even upload from Card 2; it's just backup, even for sports.
Another advantage is if you don't want to change cards much. You can shoot to Card 1 until it's full, then automagically switch to Card 2, with a quick menu right in the camera. I have my cameras set that way as default if I'm just out shooting whatever.