Traditionally, You would need a soundcard with 4 mono or 2 stereo outputs if you are mixing inside the software. This is so that you have 1 stereo out dedicated to your speaker output and one stereo output dedicated to your headphone out.
So, I would say that you need a 4-out soundcard for that. BUT this depends on how you link maschine to traktor. Can you route the audio through your PC without needing to send it through a soundcard?
The Traktor Audio 2 has 2 x stereo outs and so would do the job for you, also worth considering is the Numark DJ-IO, which is a bit cheaper but similar in functionality.
The Novation Audiohub looks like a decent bit of kit (and according to their website it is the same on the inside as my focusrite 2i4) but it requires external power, you don’t need the USB hub and it is much bigger physically than either the Audio 2 or the DJ-IO..
bob6397