Great writing, great direction, great actors. Don't mean to sound smug, but I immediately thought of The Sound Inside, The Gin Game, and Night, Mother. All were really well written, but they were also extremely well directed and had performances that were award-worthy.
It is a hard question to answer, but as an audience member, I think a two character play needs to have a tight focus, whether a specific event (e.g., Night, Mother), or a specific focus (e.g., The Gin Game and, arguably, The Sound Inside, and the characters themselves need to be immediately interesting, e.g., we learn immediately that Mary Louise Parker's character is dying; even though the play doesn't dwell on that, it is always in the back of our minds. The ramblings of a madman.