That's where there's a guy on the bottom fucking a girl in the ass and there's a guy on top fucking her in the vagina creating a triple decker sex-sandwich thus wrecking their peckers.
When someone takes a dump and gets up because he thinks he is done, only to realize there is still shit to be shat. Sits back down, takes another dump, gets up and needs yet again to sit down to complete "the triple decker".
Tony ate so much burritos in the car that he had to pull over at the rest stop for a tripledecker.