I think that many cases it cannot be helped. One 
character will move..and then the other will. 
So, there will be one cel out there that has the 
matching layers and then the rest have to be a 
combo of the moving layers that didn't happen at 
the same time. Also, sometimes I have noticed 
that the studios include all end cels, even if 
that means that both characters end up with open 
mouths, ect ect.