If you look at the VideoViewer sample it has two ImageViewerWpfControl that function in sync.
If you look at the VideoViewer- Individual playback it has two ImageViewerWpfControl that works independently, because the first uses the default PlaybackController and the other uses a named PlaybackController.
So the mechanism is to use the same PlaybackController for many ImageViewerWpfControl. Then they will always be in sync.