Double hooting is when you take a toke of weed and instead of just exhaling it you breathe out into another person's mouth. This person breathes in while you breathe out, so they are getting the marijuana smoke into their lungs. Then they exhale like normal or pass it on to another person. This is good if you're low on weed and need to stretch it out, or if you want to girls to do some mouth to mouth.
Bobby had only a few tokes left on the joint he was sharing with some of his lady friends, so he encouraged them to double hoot to make it last.