As much as I'd like to have my multi-layer cels seperated, I
don't recommend forcing it - I have two multi-layers that 
aren't stuck, but on both of them there are areas of missing
paint on some layers, and bits of leftover paint from the
layer above.  It just doesn't seem worth it unless you can
really do it smoothly.  Maybe some of the suggestions (ie
freezing) for seperating stuck sketches will work here as 
well??
Az