Hi
I assume you require the audio from the existing video clip on Video track 1 to remain as it is and play without a break.
Simply drag and drop the clip to be added to the overlay track (Video track 2) and move it to the required position. The added video will play over the video on Video track 1. The audio from the added clip will appear on Audio track 2. It can either be unlinked and deleted or more simply, muted by clicking the loudspeaker icon at the left end of the track which will then have a red bar though it.
Note that you can also move the cursor to the position where you wish to add the new clip; select the new clip in the clip bin and preview it. Under the preview window is a PLACE button. If you click the down arrow next to this you can select from the menu to place the clip on the overlay track at the cursor position.
Nat