I've run into this a few times myself, having 
bought multi-layered cels that have either been 
taped or stapled to each other or the background.

When at all possible, I remove them.  Tape can 
yellow with time & with staples, there's always 
the possibility they may rust down the road.

I haven't really had any problems with multiple 
layers moving around on me.  If you're concerned 
with them getting out of line and ending up 
stuck, you can always put a piece of a cel bag 
between the layers.  That way if they do get off-
center, you can easily move them back without 
damage.  

Good luck with what you decide to do!
--Kris

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