I think that there may be multiple video steams support which if you switch the active vehicle the video stream switches as well if the camera supports the new mavlink camera protocol. There is a videostreaming channel in PX4 Slack which has some folks there who may know the answer.
I just looked at the camera v2 protocol code and as far as I can tell if the camera support v2 protocol and returns video stream information it will switch video stream as you change active vehicle.