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Thanks -- you said "link function," so that threw me off.
If you are using that configuration, chances are your DJM-900NXS is being used in EXTERNAL mode (the audio signal is mixed within the mixer hardware itself as opposed to controlling the mix within the software), and in that case, the AUTO mode is not available. You can use the MANUAL mode and assign a MIDI control to the A/B buttons to have it switch, but for the time being there is no automatic switching for an external mixer configuration.