Double Dunking is the act of taking a shit while sitting upon another person's lap who in turn is also taking a shit. While on the toilet, there is ample room in front of the person on the bottom. This extra space can be utilized by a second person in order to increase defecating efficiency.
Variations of the double dunk include the "French Double Dunk", where the top person faces the bottom person; and the "Roody Woo", which due to its perverted misshapen entanglement of a position is very rarely performed.
Hey man all the stalls are taken, let's double dunk this bitch.