When using separate devices we try to keep things in sync but for video its unlikely it will be perfectly in sync as each device uses its own system clock and there can be slightly variable time for videos to start playing.
However, the first thing you need to do to keep two screens in sync is to apply content as Schedule. First, you add your content or media in a playlist, then assign the playlist into a schedule. Then apply the schedule to your screens.
Alternatively, if the screens are not far from one another we generally recommend going with a desktop running the chrome player and an ATI eyefinity graphics card this will not only keep your screens in sync, but you will save cost on the number of screens you'd have to subscribe.