First thing that came to my mind too! Last night I watched a YouTube video with a friend, and we did a manual 5,4,3,2,1 countdown to synch, and then text chatted via WhatsApp, video chat of one's choosing could also be used. But rolling it all into one is nice.

It would be nice to be able to do the same thing with a shared Plex server, but not sure if that would be as easy.