This conserves both time and water and can be two dudes, chicks or co-ed.
Dude 2: "Dude, I have to leave in 3 minutes and have to shit too! Let me go first!"
Dude 1: "No way! I need more than 2 minutes!"
Dude 2: "Well, looks like we're going to have to double dunk again...."
Dude 1: "Man, last night I was double dunking with my girlfriends, you know, because our water bill is so high, and since I was on top she went ahead and gave me the reach around........ it. was. AWESOME!"
Variations of the double dunk include the "French Double Dunk", where the top person faces the bottom person; and the "Roody Woo", which due to its perverted misshapen entanglement of a position is very rarely performed.