So I was having a headache with this until TechnoAddict helped me realize I was overthinking everything.
My hardware setup: I run the Traktor RCA Outs to my powered speaker Ins. I cue with the built-in headphone port in the Traktor per normal.
In "Audio Setup" I selected the "Traktor Kontrol S2 MK3 WDM Audio (WASAPI (Shared Mode))
View attachment 57177
Here is the output routing:
View attachment 57178
Make sure those settings work for you and that it cues the way it should through the headphone monitors. Remember, Traktor S2 MK3 only works with Windows 10. I had to upgrade from 7. Yes, you can still do the upgrade for free even as of June 2020.
To stream in OBS create a new "Audio Output Capture."
I named mine "Traktor Master Output."
Select the Master (Traktor Kontrol S2 MK3 WDM Audio).
My OBS scene loads in a "Desktop Audio" capture as well. Make sure you turn down that level otherwise it will double your audio.
View attachment 57179