An act in straight/gay intercourse in which two large men alternately pound the rectum of a small-framed woman/man, usually leaving the receiver with a red, swollen sphincter. The enlarged asshole resembles a gourd, giving it the apt title of a "pumpkin."
In this rare case, the damage is doubled, hence the name.
Julie approaches Barbara with a limp, nursing her ravaged body.
Julie: Last night Steve and Craig came over and jammed my bum tenfold.
Barbara: Ah, got The Double Pumpkin, eh?
Julie: Total double pumpkin action.