secret is record both (track 2) webcam and (track 1) screen recording – make your webcam take up the entire frame covering up the screen recording – then lock the webcam with video and audio track 2- drag a section of the screen recording that you want on top – hit cntrl-c – it will not copy the web cam because you locked it – now paste what you just copied to a higher track # like track 3 – higher tracks take precedence.
Repeating this allows you to switch back and forth
https://support.techsmith.com/hc/en-us/articles/360032485092-How-to-Switch-Between-Screen-and-Webcam-Video-in-the-Camtasia-Editor-Video-