Pretty sure twitch has deals with streamers that restricts how they can stream. I think SimpleFlips had a video explaining why he stopped streaming on YouTube, he didn't realize the twitch partner contract required him to stream exclusively on twitch at the time. I've also heard some things about how you can stream simultaneously, but twitch doesn't allow you merging chats from different platforms, so if you want to show chat in the stream itself, each platform might need a separate box.