Mixing from CDJS to traktor seamlessly

Yep, your friend who doesn’t want to use Traktor (and thus should be well versed in manual beatmatching) should keep on doing that, manual beat matching.

Keeping two CDJs linked and synced is possible these days with some of the later models. You’d still have to match those manually to the Traktor tracks.

Where an all software 4-decks setup would give you the great comfort of moving the master tempo and all three other tracks following synced suit, this is not possible with the setup you describe.

Plug in the CDJs as USB Midi control devices on Traktor to control Decks A & B (if you control C & D).
You will have to load his music on your computer and he will be able to use the CDJs pretty much as if he had plugged in USB sticks/CDs (selecting songs, cueing up, changing pitch, etc), except now you can sync all 4 decks. If he doesn’t want to temporarily copy his music to your hard drive, you can also plug in a USB stick that he can remove after finishing.

I’m not sure about aggregate sound cards so I’d imagine the easiest way is just to have all 4 decks (yours and his) going to 4 channels on the main mixer, and EQing from there. With this method, you’ll have to beatgrid his songs in Traktor and set up cue points, if he uses any. I’ve never tried importing beatgrids/cue points from Rekordbox’d songs.

Actually I’ve never tried this, we always had to manually beatmatch from DJ to DJ.

If this isn’t what you want.. you can try to use USB sticks in 2 more CDJs (for you), for a total of 4 CDJs and manually beatmatch. Personally trying to keep 4 decks sync’d up by beatmatching by ear is not my idea of fun…
For sync get the Nexus range and you can probably sync all 4 of the CDJs.

Failing that… if anyone knows if CDJs can be sync’d up to midi clocks, maybe they can help.