there are 2 possibilities:
1. using the cdjs in hid mode including it's soundcards (hooked up via rca to the line ins of the djm 750). you only Need 2 usb cables for the Overall Setup. don't Forget to Aggregate the cdjs soundcards before.
2. using the cdjs in hid mode and use the djm750 internal soundcard. you need 3 usb cables (and a hub for your mac). don't Forget to install the driver package for the djm and put the 2 usb cables from the cdjs on the hub and the usb from djm directly to the mac (since it's the soundcard).