You can make your own double boiler by setting one stainless steel pan atop another. The water boiling in the bottom pan heats the contents in the upper pan. While this is a handy trick, sometimes the two pans decide to stick together. One reason for this is that during the heating process, the top pan expands. This can be an issue if the two pans are close to the same size. Pans sometimes stick together when stacked and stored, and refuse to separate.
Fill a sink with hot water.
Set the stuck pans in the sink, just covering the bottom pan with the hot water. The hot water causes the bottom pan to expand.
Fill the top pan with ice. The cold from the ice causes the top pan to contract, getting smaller.
Allow the pans to set for about five minutes, and pry them apart with your hands.