I have the same problem, I might actually have bought a replacement camera unbeknownst it was the DK2 that made my old one unresponsive! It was still five years old so fine, I needed a new one anyway 😛
In any case, it acts like this for me. Boot anything using the DK2 and the tracking camera is activated. But, if I while running that try to view the webcam or use it while streaming, it just restarts repeatedly and then shuts down. If I quit out of the VR app I can then use my webcam without issues again.
If I already am using my webcam, it works just fine, and boot a DK2 enabled application... I will get no positional tracking instead, the tracking camera just will not light up, this while my webcam will keep working no issues.
I really hope this issue will be adressed at some point, right now my webcam is useless for streaming VR (not that I have any viewers) as I cannot get positional tracking at the same time 😛 The camera I got now is a Logitech C930e, before that I had a Logitech QuickCam Pro 9000 (!) which I think worked with the DK2 up to a few weeks ago.