How do you separate two glass bowls that are sticking together?

Fill the smaller bowl with ice, then lower the larger bowl into a sink filled with warm water. The smaller of the two will contract while the larger expands. Once this happens, they should separate easily.

1. Avoid breakage
Shrinking the smaller bowl and heating the larger one helps to avoid the possibility of showering yourself and your floors in broken glass. When you force bowls to separate they may grind against one another and break easily. This is exacerbated when measuring cups and other delicate instruments with handles are involved.
2. Control your temperatures
You will need to make sure that the temperatures you employ in facilitating expansion and contraction are not too extreme. Use warm water, not hot or boiling, and stick to just a few ice cubes unless the bowls continue to remain stuck together. You can also apply a lighter's flame to the base of the larger bowl for tightly controlled heat, though this may blacken glass if you hold the flame for too long.
3. Store dishes carefully
The best way to preserve your glassware is to keep bowls and other items from becoming stuck together in the first place. You can do this by practicing a simple cleanliness philosophy in which dishware is cleaned as soon as it is dirtied and only stacked for transport between rooms.