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@royalston > Yes, you just need to create an aggregate device consisting of both your CDJs. Open the Audio MIDI Setup application (Applications > Utilities folder) and click the + at the bottom of the Audio Devices window to create an aggregate, then check the boxes beside the CDJ-850 and CDJ-2000 on the right.
Within rekordbox, set the Aggregate as your audio device, then select External Mixer mode. For the outputs for deck 1, set Aggregate 1/2 and then deck 2, Aggregate 3/4. If they're backwards on the mixer, reverse them.