Instructions
Leash the non-aggressive dog and bring it into a room first. It will feel dominant if it enters the room ahead of the other dog. This also will put the aggressive dog on the defensive a little bit because the new room is not his domain. Dogs operate with a pack mentality, so one dog will have to be the dominant one. But you can help them coexist safely.
Bring the aggressive dog into the room after the non-aggressive dog has been in there for a few minutes. When the new dog comes in, it might be a little excited and want to rush the other dog. Don't let this happen; you don't want a dog fight. Lead the aggressive dog over to the non-aggressive dog and let the two sniff each other, not letting either one of them get too close to the other.
Sit both dogs down next to each other so that they can get comfortable. After a few minutes, try taking them off their leashes. Let them play a little, but once their tails stop wagging the game is over--separate them immediately. After they have been together for a few hours, dominance will have naturally settled into one of the dogs.
Give the dogs equal amounts of attention. Dogs experience jealously, so if one feels the other dog is getting more consideration it might lash out at that dog. During the first few weeks the two dogs are together, keep your eyes on them when they are out together.
How to Introduce an Aggressive Male Dog to Another Male Dog
Dog training can be rewarding and fascinating. Like humans, dogs have their own personalities and preferences. So getting a new dog and introducing it to another dog can be delicate undertaking. You must make sure the dogs are all right with each other before you can leave them alone together. This is particularly tricky when you have to introduce an aggressive male dog to another male dog.