We recently adopted a Rat Terrier/Lab mix who is very energetic, sweet, and playful. We've had her for about two weeks now and she seems to be adjusting to life in our house well. However, we got another dog today. (Or should I say, we brought back a dog that we used to have. ) He is very sweet as well, except he is only sweet with humans, and when it comes to other dogs, he get's a bit offensive. He growls any time she gets close to him, and she doesn't do anything specifically, just follows him around and sniffs. Is there any specific way I should go about introducing them to each other?