Because Traktor can't see two sound cards at once, and the DJM-2000 reports itself as two cards, if you want to get the audio back into Traktor, you need to aggregate the cards and in Windows, you do that with ASIO4ALL (get 2.9, 2.10 has some problems).
This video covers some of that in Mac...
If you aren't needing to route the audio back to Traktor, you don't need the aggregate as the other application can pick up the input soundcard. Just be sure to open your DJM-2000 configuration utility and set it for 6/2 mode.