I’m afraid there’s no reliable way to do this right now. I’m working on it for 4.16 though.
You could try to enable the “Preache File” option on the FileMediaSource, open the two videos in both players, and once they’re opened, call Play() on both players.
You will have to disable the “Play On Open” checkbox on the media players. Instead you need to listen to the OnMediaOpened event in your Blueprint and only execute Play after both players are done opening the media.
Assuming that both videos were created with the same encoder settings and frame rate, they should stay more or less in sync.