When two people have to poop simultaneously, so one person sits the the lap of another while facing each other on the toilet. If it’s a boy and girl and they are having vaginal intercourse while double decking, it’s called docking the buses.
Yesterday my bro and I had to both poo, but the spare bathroom was close. We ended up doing a double decker, luckily the bowl didn’t fill up.

My girl and I had to drop a deuce so we double decked. She’s got them big boobs though, so after a minute we docked the buses ;)
by MelonLord95 April 9, 2022
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The act of inserting two penises into one vagina, a deft pun on the phrase double deck her.
Yeah, we did a double decker on her.
by Anarcho Crust Fuck April 5, 2020
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When a person stacks one fist on the other, and aggressively shoves them into a woman's vagina.
"I'm still hurting from his double decker last night"
by Kelsey15 March 8, 2014
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Similar to the Upper Decker, the Double Decker involves taking a shit in the victim’s toilet, not flushing, then repeating the process in the tank. The end result is the unsuspecting victim flushing the lone turd, only to be accosted by a stream of liquid feces into the bowl. This is far beyond an innocent prank, be sure the victim deserves it, as there’s no going back.
Allen was house-sitting for Sam and had a party at his house...he was only caught because somebody left a Double Decker in Sam’s toilet.
by AlienMan1964 February 7, 2018
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A Double Decker is the action of one removing the top of a toilet, and proceeding to take the actual fastest shit in the top toilet, and leaving it floating like the fucking Titanic.
Double Decker are usually meant to either torment or assert dominance by defecating inside of the toilet.
My friend always asks to use the bathroom, every-time I say “ Don’t shit in the top of my toilet”.
Then he says “I’m not going to”.

Then he Double Deckers me, and I find it a week later because my bathroom develops the most malevolent smell.
by Djent Bro! April 5, 2022
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To fold ones erection back on itself then reinsert to continue coitus.
I totally double-deckered my girlfriend/boyfriend last night.
by Stephan Noir February 4, 2021
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When you take a shit in the toilet tank instead of the toilet bowl.
Guy 1: Dude I just took a Double Decker!

Guy 2: Aw no way man, right in the hotel bathroom! Right on!
by Triangle smacker December 30, 2020
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