When you take a Burger King Buck double and split it at the pattties and put another Buck double inside the two meat patties with buns to make one singe sandwich out of two buck doubles.
During sex, when a man and a woman share a double sided dildo, thus simultaneously penetrating the anus of both individuals during vaginal intercourse.
I had a Buck Double last night. It wasn't at all what I expected, and I can't believe they actually advertise it in public.