How to separate two jammed-together measuring cups?
Filling the lower jug with water and then putting the whole thing in the freezer worked. I checked it after a couple of hours and didn't think it had worked as I still couldn't separate them, but it mustn't have finished freezing at that point because when I looked again a bit later the ice had travelled up between the two jugs and then at some point the inner jug must have popped free.