. . Forward one frame(Left). . (Ctrl+PageUp)

. . Back one frame(Left). . . . (Ctrl+PageUp)

. . Forward one frame(Right). . . (PageUp)

. . Back one frame(Right). . . . (PageDown)

To use these keys, ensure that the keypad 'Num Lock' is off.

These keys allow you to displace one movie relative to the other in order to effect synchronisation.
The title-bar of the window displays starting, current and last frame number of the left and right movies together with the current width and height of the frame in pixels.
When 'Ctrl+PageUp' is pressed, the left-movie advances in time by one frame and the start and current frame numbers increment by one. The right-movie last-frame number decrement by one.
You may like to visualise this as snipping a frame off the left of the movie strip, lining it up with the start of the right movie and snipping a frame off the end of the right movie so that both 'clips' are the same length.
In a similar way, 'PageUp' snips a frame off the start of the right movie and a frame off the end of the left movie.
If 'Ctrl+PageUp' has previously been used on a movie to increment the start-frame, 'Ctrl+PageDown' will decrement it (effectively putting-back a frame), otherwise it will have no effect.
The last-frame number of the other movie will be incremented to increase the 'clip' length by a frame.

Moving backwards through the movie will be slower than moving forwards so there will be a slight delay before the action has any effect.

The difference between start-frame and last-frame numbers, for each movie, gives the'clip' length and will be the same for both movies.