I have only setup hid MIDI once for a friend to test Cross on some cdj900 on a mac
Here is how to setup
http://support.Apple.com/kb/HT3956After you have setup your mac aggregate audio so Cross can see the two CDs 850 as output 1&2 and outputs 3&4
And this vid will show you how to setup a sound card for external mixer mode....
http://thedjpodcast.com/episodes/crossdj-soundcard/You will have set The drop down box ,where you choose you sound card .....choose the mac aggregate audio device as the sound card
And set up outputs 1&2 and 3&4 for the two CDs
Plug they two CDj into the mac by USB.,.Set the CDs 850 to p.c mode
,.,check to see if you are running the right firmware inside your CDj 850