On Sun, 15 Feb 2015 13:03:36 -0800 (PST), wrote:

Best way to unstick is to put some ice cubes in top container and place the bottom container in hot water. Changes the air pressure in "vacuumed" area and they come apart with some twisting.

Best way is to prevent them from sticking together early on. Several
ways to do it without all that effort, wasting hot water, ice cubes
and twisting.