Normally you would choose the "Optical Remote" option on the camera's menu to do this, even though you're not actually going to be controlling the flashes optically directly from the camera. This is because the camera sees the YN-622C transmitter as a camera mounted external flash. According to the third party written "The Other YN622C Use Guide", the newer Canon bodies such as the 5D III, the 1D X, the 6D, the 650D, (and presumably the 70D introduced after this manual was written) will function correctly using the YN-622C when set to "Optical Remote". You would need to use the menus on the flashes themselves when doing the initial setup to designate which flash is in 'Group A', 'Group B', or 'Group C'.

Here's where it gets a little murky. Both of your 430EX II flash units are compatible as optically triggered slaves, but not as optical masters. The instructions linked above seem to be saying that the YN-622C system will still allow you to separate them into groups and control the ratios between them, but some settings will need to be made using the controls on the YN-622C itself instead of using the camera's menu.