I went to a professional framer with a two layer 
cel and matching bg, and asked them what options 
they could come up with. I mentioned bags as 
well, but after some thought we took this 
approach: They framed the cels so that each 
layer was separated from the other by a couple 
of millimetres, and the background was also 
separated in this way. No bags were involved, 
just matts at the very edges of the cels (so the 
thickness of the matt layers was what kept them 
apart. Incidentally, the matts hold the cels in 
place with pressure, no fluid or other 
adhesive). This multilayer setup was then framed 
with archival glass, and it looks great ^^ The 
tiny spacing between the layers created a slight 
3-d effect as well.