[yorkietalkjilly]

I don't know if this applies to your dogs' specific problem as there could be several reasons they behave this way when together but, based on limited information, here are some thoughts to consider and possibly use some of the techniques if you think any might help.

Dogs are often super aggressive on a leash because they know they are restrained and confined to the length of the leash and don't really feel free to protect themselves should another dog attack and perhaps walking them together makes them go into pack mode - ganging up to warn any and all approaching dogs. Dogs also know that their own packmate, restrained very near him, might redirect his aggression to him and bite if restrained by the leash from freely protecting himself due to the leash, so they are often anxiety-filled if another dog gets near when leashed with another dog in close quarters. They often pick up on an approaching dog's own sense of aggression, since that dog, too, is likely also feeling constrained and sending out "I'm tough" vibes they feel the need to answer to. Dogs don't care that another leashed dog is just as restrained as they, so those feeling vulnerable, not trusting their pack leader human to be able to take control of any situation, might send out aggressive body and oral signals to warn other dogs to leave them alone - "I'm so tough - don't you dare come after me, see?"

Gaining more respect and getting your dog to recognize you are in control and he's totally safe when anywhere around you will do a great deal to help him feel he doesn't need to feel anxious or vocalize warnings - that he's safe and you will keep him that way. You might want to start him on the Nothing In Life Is Free program so that he sees that you control all his resources and his life and teaches him to always look to you for everything he needs, even safety. Dogs instinctively love working for their resources and rewards so they really get in to the fun of performing a command and waiting to be invited as part of the NILIF program and begin to learn you are always in command of all situations and resources.

Here's a couple of ways to show your dog what will happen unless and until he learns to control his impulses and instincts and stays calm when another dog passes. Anytime a dog out walking on the leash begins to vocalize his warnings as another dog or person approaches, say "uh oh"(during training, "no" after the dog has learned the lesson in a couple of months), immediately turn him and walk very fast in the opposite direction and keep walking away from the other dog until your dog quietens down. When he goes quiet, that's a signal to you that he's calmed himself so you can treat him if you'd like, turn and walk back as a signal to him that calming down gets him what he wants, going forward toward the other dog and sometimes even a treat. But then immediately "uh oh" and turn, walk back again if he starts up with the barking or growling, which he will do until he's got the hang of what's going on - which usually takes about 2 weeks for the dog to begin to see the pattern. After more training and repetition of the above training, he'll begin to police himself and calm himself down occasionally to avoid having to walk away from the dog he wants to get closer to. After a couple of months, he should be getting pretty good at staying calm when another dog approaches. If not, pull out a piece of boiled chicken and stick that right in front of his muzzle until the other dog passes with him staying quiet - then give him the chicken as a reward, put in back in the treatpouch as you turn the other way and walk away if he doesn't. Or you can walk him in a very fast, tight circle, so fast that the dog has to concentrate only on keeping his feet while the other dog goes past. He'll slowly learn that other dogs can pass without trying to attack him and that, even if they aren't trying to attack and your dog just wants to get to them to be friendly, he's still got to behave as you want to keep moving forward and avoid your corrective measures.

Crossing the street and putting as much space between your dog and any approaching dog is a great idea and will lessen his anxiety and help him deal with the stress of worrying about whether the approaching dog will go after him until he's learned proper walking behavior. Walk very fast to keep his mind very focused on the walking than the approaching dog - run, urging him forward with "Let's go, Buddy!" in an upbeat voice, if you need to in order to hold his attention on you - until the other dog is past.