Two clips selected. (right click drag to select both clips)
The left side of the clip is synced and both edit as expected. (making the clips shorter left to right)
the right side is not synced and only one clip at a time will edit.
 
Also how exactly does "From Gallery / Upload" work?
 

I have noticed that when you edit two clips or tracks at once you have to move them enough so that their edges line up at some point. Then they will stay in synch.  So if you select two clips and grab one of their right edges, say one that is shorter than the other, then you will need to move the grabbed edge out far enough for it to be equal to or greater than the longer one. Once it gets to that point the other clip should start moving in synch with the grabbed clip.