Higher video tracks take precedence over lower. One approach...
When you want to see the lower (main) track video, just split out and delete the higher track video during that segment, leaving a gap. Video will cut from one track to the other.
Below, a fade has been added between the tracks. Track two fades to track one, then in a few seconds fades back to track 2.
The first fade uses the X fade transition. The second uses the F fade button.
If a separate audio track is disturbed when the video excisions are made, first right-click in the area to the left of the track and LOCK TRACK.
Press <ctrl-Z> (EDIT | UNDO) if you make a mistake, to undo step-by-step.