Quoted from tjw998:Try dunking the outer one in hot or boiling water while the inner one is full of an icy/water and salt mixture and let them sit there for a few minutes. Should help break the bond between them.
This. Thermal expansion (along with contraction in this case) is your friend to break the seal.
Penetrating oil would likely work too, but I personally wouldn't want the stuff anywhere near a food surface.