Reply to cdj-900s/xone 42 with traktor

hey guys, I was wondering I am required to use a audio interface if I want to hook up my cdj-900s to traktor while using an external mixer? I am capable of hooking up both cdj-900s to control the decks, but I am unable to get each deck to output to a different channel on my xone 42. They both seem to play into channel 3. Am I doing something wrong in the settings, or is it not possible to do this without a audio interface?

Thanks