@marvin > Traktor doesn't allow for the CDJs / DJM to be used as separate audio interfaces; you need to either create an aggregate device (Mac) or use ASIO4ALL (PC) so they look like one single audio interface to the software.
This video has some information (old, but relevant) for creating an aggregate device and configuring it within Traktor:
...but my recommendation would be to use ONLY the DJM as the audio interface (as such, no aggregate / ASIO4ALL is needed), but you need to ensure you have Traktor configured for EXTERNAL MIXER mode.