You can start two separate recordings, especially if you have an external screen like I do.
In Quicktime, first you start a normal video recording using the web cam (in my case the built-in cam), move the window to the secondary screen, then start a screen recording on the main screen, not showing the webcam video window.
So you get two separate recordings and you can edit them together afterwards, enlarging or minimizing the webcam video as needed.
Not sure if both videos would record sound from the same mic. I didn't test. In my case the webcam recorded the internal mic, and the screen video recorded the mic plugged into a HQ USB sound card.