1

When using two playback screens, after bringing two videos to same position, joint control play button resets one video to beginnning.  So, common playbay does not begin at desired frame(s).

2

Do you see the red markers corresponding to the time origin in each video timeline and in the common timeline?  (this is the synchronization frame in each video).
If the synchronize button does not work, does it work if you instead set the time origin independently in each video?

3

Yes - I do see the two markers after jogging each video to a common position. But when the joint play button is selected, one video snapps back to the video origin before both play. So, they are no longer sychronized to the same start position.