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1.
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.
by John Price April 10, 2008
 
2.
When one person sits on the toilet to shit and another person sits on their lap to shit. The top person must do their best to aim for the small space available that people normally just use to look down at their nuts and watch their shit float around.
This conserves both time and water and can be two dudes, chicks or co-ed.
Dude 1: "Wow, I have to leave in 5 minutes and gotta shit so bad.... this is going to be a fast one!"
Dude 2: "Dude, I have to leave in 3 minutes and have to shit too! Let me go first!"
Dude 1: "No way! I need more than 2 minutes!"
Dude 2: "Well, looks like we're going to have to double dunk again...."

OR

Dude 1: "Man, last night I was double dunking with my girlfriends, you know, because our water bill is so high, and since I was on top she went ahead and gave me the reach around........ it. was. AWESOME!"
by drgstrSMB April 10, 2008
 
3.
A sexual act where one person deficates into another person's anus, then the reciever then deficates into the original peron's mouth and then eats it.
I want you to give me a double dunk!
by Billy March 07, 2005