You could record both at the same time, and then edit the video afterwards to switch between your desktop screen and the webcam video.
There are a lot of applications for Mac computers that will let you either record from the webcam or record what is happening on the computer screen. Many video editing tools will let you stitch the two together.
Theres quite a bit of competition in this business field I am getting into, and all of the competition have the ability of recording themselves and then being able to show what is on their screen entirely without showing themselves or anything so the software of doing both live and at same time is out there. Just don't know what it is and if its mac accessible. Theres many youtube videos for this field of the competition doing it so I don't know.
I need help guys!