(transitive verb): The act of having sex with two women at the same time. Technically this refers to instances where the women are stacked on top of each other, stomach-down. Colloquially, however, double-stacking is used to mean any act of a threesome where one or more men have sex with two women at the same time.
Bro, look at those two fine-ass hotties. Let's double-stack those bitches and take their temps.