Run webinar with more than one webcam
Webcams live in the universe of being devices plugged into computers, and switchers live in the universe of standard video sources being plugged into them.  These two universes only talk to each other via specialized interfaces (if at all).

Happily, there exists software that gives you switcher-like functionality as a software package for your computer.  One such package is manycam.  There are others you can find.

Trying to get webcam video out of your computer as video, into a switcher, then back into your computer so that it can be webcast is about 3x more difficult than it needs to be.  And it requires a lot more space on a physical desktop.