The double decker is a combination of the upper decker and a blumpkin. The guy is on the top tank performing an upper decker while the girl faces the wall with her feet on the toilet seat giving him a blumpkin while she drops a deuce in the bottom tank
Double Decker The bitchpissed me off so in her bathroom i gave her sister a Double Decker
When taking a shit, having someone sit on your lap aiming their anus between the first person's legs. Both people shit at the same time, creating a "double" effect and saving water in the process. For utmost efficiency, time the splashes similar to Olympic synchronized diving.
She sat on my lap while I was on the toilet so we could double decker that shit.