So you are visiting another streamer's stream, how should you act?
Just because you are a fellow streamer does not give you permission to act any differently than any other audience member in chat. Whether you are leaving to do your own stream or just chatting about streaming in general, never out right talk about your streams unless explicitly asked by the streamer.
While yes, you may encounter instances where a streamer and a chatter are talking about the chatter's stream you have to assume either A) that chatter is a close friend of the streamer where that type of behavior is acceptable or B) the streamer has asked the chatter to talk about their stream.
If you have to leave to go do your own stream and you want to make sure the streamer knows you are leaving you can just let them know, “Hey Streamer, I got to head out for now, but thank you for the stream! You were great!”
You want to grow and become a bigger streamer? Never discuss your own streams in other people's chat without permission. This will leave a bad taste in the mouth of the streamer and by extension their community. That's one mistake you do not want staining your brand and reputation.