When one person sits on the toilet to shit and another person sits on their lap to shit. The top person must do their best to aim for the small space available that people normally just use to look down at their nuts and watch their shit float around.
This conserves both time and water and can be two dudes, chicks or co-ed.
Dude 1: "Wow, I have to leave in 5 minutes and gotta shit so bad.... this is going to be a fast one!"
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!"