Put ice inside the inside one and set them in a sink of HOT water so the water only touches the outside bowl.

Gosh, this seems like it should have worked, but it didn't. I even tried the reverse (hot inside bowl and sitting the outside bowl in ice. Yikes. I think I'm going to try filling the inside bowl with water and then putting it into the freezer until it freezes and then put it into sink of hot water. Last try. This may have to go into the trash.

You might try spraying WD-40 between the bowls and tapping with a wooden spoon. If that doesn't work, new bowls. If you still want to nest them to save space, wax paper between them will prevent them from getting stuck again.

Hooray! I filled the top bowl almost full with water and froze it. I used two trays of ice cubes on the first try. Then I put the bowl/s in a larger pot and filled boiling water into the pot to below the rim of the bottom bowl. I waited awhile and gave it a try. Didn't work, so I put it back and waited some more and then when I gave it another try they slowly came apart. Thanks aliceinwonderland! I'll store them with their lids on. Disaster averted.