When two friends defacate at the same time in a public restroom and hold hands under the stall.
James: Hey Steve is that you in the stall next to me?
Steve: Yeah man.
James: Are you taking a number 2?
Steve: You bet. Are you?
James: Yeah we should hold hands and take a number 4.
Steve: Omg Yeah!
It is a combonation of urination and defacation in the same restroom visit. Derives from from the words number 1 and number 2.
James: Hey Travis you were in there awhile. Did you do a number 2?
Travis: No man, a number 12.
James: Why do you give it a special name? Everyone pees when they poop most times...