@Matthew > If anything, I would connect the drive direct and connect the players both via the UBS hub. This will provide a dedicated port for the hard drive which requires a more sustained data transfer rate than the CDJs do.
The first step you should take is to change the USB connections (as above) then once connected, load the Audio MIDI Setup app (under Utilities) and see that both CDJs are shown as MIDI devices. From there, open Traktor and run your Setup Wizard (under the Help menu) and configure it to use the CDJ-400's. Once that's complete, select PC MODE on the CDJs and you should see the prompt to select which deck each controls.