Plex can play to multiple sources at the same time and works well for that but it doesn't do simultaneous playback to multiple devices (i.e. in sync over multiple devices for true multi-room).
It's been a feature request for donkey's years but I wouldn't hold your breath on it, the plex business plan appears to be introducing the features that they think people want, not the features they actually want.
Plex can play to multiple sources at the same time and works well for that but it doesn't do simultaneous playback to multiple devices (i.e. in sync over multiple devices for true multi-room).
It's been a feature request for donkey's years but I wouldn't hold your breath on it, the plex business plan appears to be introducing the features that they think people want, not the features they actually want.