How to play video at the same time on separate computers

VideoLAN VLC Player has options for broadcasting a/v.

You can stream the media by opening VLC Player and clicking Media > Streaming. Then set your options. Use the streaming section of VideoLAN's site if you need help.

Once you're streaming your video, you can connect the other device to the stream (probably using VLC, as well). As long as you've selected "play locally" it should be playing on both devices at the same time.

Hope this helps.