You don't mention what sex the dogs are? Are both dogs usually ok with other dogs? There are a few reasons why some dogs don't get along, one of my fosters at the moment is growling at my sons bitch - but it's because she is fast and "spinny" and he needs to control her movement. Other collies seem to be aggressive to each other (family members) but its all neck grabbing incitement to play which may be misconstrued as aggression if you've not seen it before.
Old vet told my mother, Valium,. Take some yourself put the two dogs together and walk away, they will work it out. They were dogs not bitches mind.
Take them to a neutral area that is new to both of them and walk them a few times, this makes them feel as though tbey are part of the same pack.
Then introduce them on the less aggressive dogs patch, the person owning the dog that's at home always enters the meeting area in front of the dog (walking in front shows dominance) then add the new dog, and if either of them go to start scrapping grab them and lift them up, don't drag them backwards dragging a dog back makes them more aggressive. An no shouting, that heightens aggression too
You're the boss and if you show them in a firm calm way that this fighting sh!t will not stand, it usually works. An alpha dog won't normally let other dogs fight, so you need to be the alpha dog.
Take them to a neutral area that is new to both of them and walk them a few times, this makes them feel as though tbey are part of the same pack.
Then introduce them on the less aggressive dogs patch, the person owning the dog that's at home always enters the meeting area in front of the dog (walking in front shows dominance) then add the new dog, and if either of them go to start scrapping grab them and lift them up, don't drag them backwards dragging a dog back makes them more aggressive. An no shouting, that heightens aggression too
You're the boss and if you show them in a firm calm way that this fighting sh!t will not stand, it usually works. An alpha dog won't normally let other dogs fight, so you need to be the alpha dog.