mostly occurs the morning after a heavy drinking session. When someone is sitting on the toilet clearing their bowels and they suddenly have to throw up as well - both "exit pipes" are in full flow as vomit and excrement both leave the body by their respective routes.
Usually accompanied before and after by groans of "oh god" "never again" or "oh shit" and requires someone else to clean up the mess as the double dumper is too worn out to do much more than wipe their ass.
did you here that ? sounds like "Johny" has just done a Double Dumper. I ain't cleaning that up