I remember reading a science fiction short story in the 1980s about two mirror universes, where the same incident is going on at the same time. In one universe, the keyword is Sleep and in the other, it is Aestivate. The story ends with the two beings interchanging their respective universes.
I vaguely remember that in the first universe, the being is asked to Sleep and in the mirror universe, the being is asked to Aestivate. Towards the end of the story, there is a flash or something, and the two beings realise that they have switched their universes