Dueling Harmonica 

Two individuals, undefined in orientation and/or gender, each partaking of one half of a mans penis, divided lengthwise, by means of oral suction.

After a heated arguement, they decide to compromise and perform the dueling harmonica.

Guy 1: Yo, these two babes shared my donicker last night.
Guy. 2: You mean they gave you a dueling harmonic?
Guy 1: Yeah, I guess.

Dueling Harmonica by The College Board April 14, 2008

Dualing Harmonicas 

The act of two women performing oral sex on a man, while both women slide their lips up and down parralel sides of the shaft.

That threesome was crazy, jill and leilani went dualing harmonicas on me!

Dualing Harmonicas by johnny longshanx March 28, 2011