Output video on two different windows
I got that idea while running a karaoke session. Sometimes the person running the karaoke is not able to see what's on screen (screen's on the side, or behind the user), might want to sing and participate but can't follow what's going on. Also, that person can't see if something's wrong with a karaoke.
An idea to fix this would be to spawn a second mpv and send it the exact same commands.
This would duplicate I/O and the computer running has to support it (as well as decoding two videos at the same time) so this would be for advanced uses only. That second window would be movable like the first, main one, but would output no sound at all (or maybe sound on a different device?)