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In XSplit, you can add any service as a custom one. I don't have one handy right now, so I'll write it from memory. Log in to Twitch, go here, and click "show key." Copy the key. In XSplit, select "custom" in the stream service settings. In its settings, enter rtmp://live.justin.tv/app in the service address (possibly without rtmp://, just live.justin.tv/app), and the copied key in the key field.