You've seen the double finger flip, one pointing up and one pointing down.  Here's the gesture (performed by the thug on the right), followed by the particular act it refers to.

Cain & Gabe by Straight Naked Thugs
Cain & Gabe (Straight Naked Thugs)

Kyle Thomas & Josh McKenzie by Badpuppy
Kyle Thomas & Josh McKenzie (Badpuppy)

The thug on the left displays the double finger flip in which both fingers point inward.  That, of course, refers to Dick-2-Dick sex:

The Fisher Twins by
The Fisher Twins (