Hi
Being able to set a frame marker fixed to a point on a particular track to enable clips on different tracks to line up is a suggestion recently made to NCH and is something they may get round to looking at. In the meantime you could try lining up specific peaks in the sound tracks.
Another approach that you could try is ..
Once you have the videos set up to play on the same sequence view...e.g. in a split screen,
Set a bookmark at the start point on upper track (it wont be fixed to the track however )
Now play the main track to the required point.
Set another Bookmark.
Move the red cursor line out of the way.
Grab the main track at the bookmark position and drag it to the first bookmark.
If this is done carefully the clips (L and R) should be in synch.
Nat