Using FaceTime and Skype (both open at the same time)
There is an app for that :)

ManyCam will allow you to use the iSight camera over multiple applications. I haven't had the chance to test it myself, but according to the app description, it should work just fine.    

Some customers complain about a slow Mac, some don't. So please give feedback about this app. 

EDIT:
CamTwist should do the same.