Curently, EasyRTC doesn't support multimedia streams in a single call.
Some people have gotten around this by having a second client running inside an iframe.
We will be supporting multiple media streams in the future but we're waiting to for the new screen capture API to come out, because the existing one isn't useful for anything other than capturing an image of your screen, it's proof of concept not something that is usable in a production environment. If you search the group, you'll find this topic being discussed.