A derivative of the devil horns made popular by Ronny James Dio. A Double Viking is giving the sign with both hands with as much enthusiasum as possible.
In the same family as the "too much metal for one hand" sign but a DV is better in that one can more easily toss it up without worrying about putting their hands together. Thus making it far more deadly.
"Hesher Mark was so excited about the rock show that he tossed up a Double Viking in excitement"
If you were to illustrate it, a Double Viking would look like th following. \m/ \m/