How to reinforce watchdog behaviors?

I have two really friendly pit bulls. I don't think they would bite anyone either...but I want people to think they will. So, only one of my dogs barks, when someone is outside or ping by. When my dog barks at someone or something outside, I encourage it. "Good boy", "Who's that", "Get them, tweet" (Tweet is his nickname), anything to encourage the behavior, even giving him a treat for it. Have a friend come over and knock really violently on the door or ring the bell a bunch of times, and when your dog finally starts barking, give the same encouragement. But don't have the friend come inside, and don't open the door. Be sure this is a trait you want your dog to have though. Because it has to be all or nothing.