Premiere and Vegas don't really do realtime capture. They can capture to a file to edit later. But each stream would be a different file.
Now once you have the 2 videos you can easily create a new video with cuts from camera 1 to camera 2 without changing the audio feed.
To actually capture and manipulate two video streams in real-time you would need two cameras and an audio/video switching device and somebody running it who determines which feed is the "live" one. This is more like TV broadcast than video editing.