Used in the exchange of drugs, a double hand shake is where you shake both of the dealers hands in your opposite hands, i.e right with right, left with left, and swap over the goods.
He takes the money out of your right hand whilst you take the dope/MDMA whatever with your left.
Used to disguise the transaction.
Dude you got my dope?
Yeah let's double hand shake so that guy over there doesn't see.