Bill,  I have tried several different ways ( soaking in water, heating in
oven, steaming and combinations of all of the above) and still the best
to me is to use a sharpened putty knife on the glue line on one side at
the gasket end.  Hit with a rubber mallet until it starts to separate,
then reposition blade in separation on gasket end and drive forward to
the narrow end to finish separating two half's.  They will not always
split perfect but will glue back together very well.

I have not tried letting them freeze first to see if they will split
better, but I think I will try it once to see how it will work!

Brett Mohr