Streamers you follow appear in your following list when they are on another person's stream
The idea is if a streamer you follow is playing or on a stream you don't follow, it would show in your following list the stream they're on. It would allow people to watch streamers they like when they're not on their own stream, and perhaps even discover new streamers they like.
As for the logistics of such a feature, it could require the streamer that is live to send a request to the guest on their stream, and that person would have to accept it, then it would show the guest as live on another person's stream to the guest's followers.
To prevent exploitation, perhaps the ability to send a request to a guest could be limited to people on the live streamer's friends list. This would allow the live streamer the option of whether or not to send the request and alert the guest's followers, and of course allow the guest to deny alerting their followers that they are on someone else's stream. Then, the streamer that is live could resend their request at any time during the stream, and the same for the guest. Then it would stop alerting the guest's followers once the live streamer goes offline, and they would have to send another request.