Official comment
You need to use ASIO4ALL to "bundle" the audio drivers for use with Traktor as it can only identify one audio interface at a time.
This video is made for Mac, but the concept is similar; an aggregate device is created and then selected in the software. With ASIO4ALL, you don't need to aggregate the devices, it will do that automatically - you just select ASIO4ALL as your output in Traktor.