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2.
Taking a dump into the toilet tank so when a person flushes the toilet they are astonished to see a turd come down.
Marlene was shocked at the Omega Mu House party when she flushed the toilet and a big nasty frat boy turd came down and she realized that she had been the victim of double decking.

(WARNING: Do not try this. You could easy break a toilet. )
by Zoro the Ass Bandit April 02, 2013
 
1.
The act of releasing a bowel movement on top of another bowel movement.

Double Decker
Bismark
It was no surprise that Steve was double decking in the public bathroom.
by Frank the Tank 2 August 06, 2011
 
3.
you and a friend both fucking girls on a bunk bed in a synchronized manner
Bob and Alex double decking dimes on bunk bed in tune
by Rage-hard and no regard November 18, 2011