Instructions
Adding New Dog
Before attempting to add a new dog, your aggressive dog should be on perfect verbal command. He should be able to stop, go to a spot and come on command as well as focus on you rather than reacting to dogs on the street or in the home. If your dog doesn't have these commands perfectly, you won't be able to trust him with a new dog.
Take the two dogs on a walk together. Keep them from interacting but allow them to smell each other's urine. This allows them to get some information about each other and walk together as a pack.
Bring them home and keep them in separate rooms, separated by a door, not a baby gate. Every few hours, switch the rooms so the dogs can sniff each other's scent but do not let them see each other.
Continue this arrangement for at least a week, adding additional time for dogs of same sex or similar age. Take the two dogs on walks together every day.
Observe your aggressive dog's body language around the new dog. When he is calm, take the two dogs to a large field or fenced area in a neutral place. Let the new dog off-leash and keep the aggressive dog on a 20- or 30-foot line so that you can grab him if necessary but he doesn't feel trapped like he would on a 6-foot line. Before you allow them to interact, rub a towel between the hind legs and on the rear of the aggressive dog. Rub this towel over the resident dog.
Continue to keep the dogs in separate rooms until you have several positive interactions in the neutral setting. Then place a baby gate between the rooms so that the dogs can see each other all day. After at least a week of this, you can begin to keep them in the same room for short periods. Build up to allowing them to spend the whole day together.
How to Introduce a New Dog to a Pack With an Aggressive Dog
Adding a new dog to your household is always dicey proposition, especially if you already have an aggressive dog in the mix. Not all dogs get along, so if you have an aggressive dog, be aware that the situation may not work out. Introductions are more likely to be successful if the new dog is a different gender from the aggressive dog or a significant age difference exists. The least chance for success is two females of similar age; if this is the case, add additional time to each step.