Youtube has multi-stream capability and I've seen one streamer use it years ago to let you pick which multiplayer user to spectate. I assume they used separate streams inside in the .m3u8 file used for HLS, much like how you'd do different resolutions. Haven't seen it done in years.

.mkv files natively support adding an arbitrary number of video tracks, though player support is kind of rare. (it's _ by default to cycle video tracks in mpv)