You know that Spiderman meme where it's three Spidermen pointing at each other? This is the same idea. A "whodunnit". The thought behind this is a level of aggression, with the finger guns. There's also an implication of a lack of seriousness. The two hands are blaming each other but aren't threatening enough to escalate further. 
 
It's additionally been suggested that there may be a sexual nature to the pose. If that's your interpretation then who am I to dismiss your reading?