In both short stories, the use of you is a hint that there is an unreliable and possibly not quite sane narrator. In each case, the second person point of view is used in unusual ways. Second person is by far the rarest point of view.
In “The Cask of Amontillado” by Poe the use of you is intended to be disarming. In the beginning of the story, the narrator directly addresses the reader as an intimate friend:
The thousand injuries of Fortunato I had borne as I best could, but when he ventured upon insult I vowed revenge. You, who so well know the nature of my soul, will not suppose, however, that gave utterance to a threat.
Throughout the rest of the story, he addresses Fortundao with you, reinforcing the idea that they are just two friends going on a stroll. This is intended to put Fortunado at ease and off guard, so he will be easier to kill.
In “How” by Lorrie Moore, you is used as a character, a woman. This unusual use of you breaks assumptions about the role of the reader and the use of the pronoun. This intimate and unusual use of you makes the reader part of the story, whether male or female, but also makes it harder to discern the reliability of the narrator.