Elaine and Kramer are left alone a few times. Off the top of my head, in "The Limo", Kramer tries convincing Ekaine that Jerry is the head of the Arien Nation while waiting to be picked up in the Limo.
In another episode, Elaine and Kramer decide that they have sex 5 times a week in the fictional relationship they're going to pretend to have for the benefit of her psychiatrist. I'm sure there are others.
Elaine and George are left alone in the episode that Elaine talks dirty into Jerry's tape recorder. One of the all-time classic scenes has Goerge and Elaine sitting in his car as he drops her off at home.