First, fill the inner bowl with cold water. Now fill your kitchen sink (or a washing-up bowl large enough to accommodate the glass bowls) with hot water. Finally, float the stuck bowls in the hot water and press down so that as much of the outer bowl is submerged as possible without getting any hot water between the bowls.
The outer bowl should expand slightly whilst the cold water should stop the inner bowl from doing so. If this doesn't work after a while, tap the inside rim of the inner bowl from the inside whilst applying gentle upwards pressure.
Let me know if this works: ace at blueyonder dot co dot uk.
RLL (Gloucester)., November 2007