The co-streaming is really nothing new. You just would use multitwitch.tv

The problem with Beam, or now "Mixer", is that hardly anyone uses it. So, if you're trying to get in and make it, so to speak, as a streamer, it's not as populated as Twitch, causing it to be just as hard to draw numbers into your stream.

Maybe this will help Beam become more popular, but highly unlikely to make it serious competition to Twitch.