I can't speak with any real authority here but... WebRTC requires the two end points of the connection swap information for networking and things like what code

I can't speak with any real authority here but... WebRTC requires the two end points of the connection swap information for networking and things like what codecs they support. I don't believe the MediaStreams between a->b and a->c would have anything to do with each other. They only share a common VideoSource. That would lead me to believe that it would try to encode two different ways if that was the only option. More than likely you'd probably have the vendor select a preferred single codec instead of doing things differently for each user, though that would be pretty neat if it wasn't too resource intensive.