Fill the top glass with a little ice, and then put the bottom glass into a sink or bowl of warm water. Give it a second or two to work, and the difference in temperatures between the two will cause the bottom one to expand a little and the top one to contract a little.
Cooked up,. Put them in hot water. Fill the top one with hot water, this should make the rim of the bottom one expand. Aloha!
Mogie gave the right suggestion. You want to expand the bottom piece and contract the inner stuck piece. I put the lower one in hot water for a bit, then fill the top one with ice water or have a bowl of ice water and put the top of the stuck one into the ice water, but not so that it will cool down the bottom piece.