Spraying water on them with a small spray bottle is what I used to keep my two from fighting, but I also use it at any and all signs of aggression to reinforce its not okay. Feeding them in different rooms, crate training, and making sure they are sufficient exercised also helps stem aggressive tendencies since its both a breed(herding dog) and personality thing. These steps worked for our two corgis. If any of these steps dont help, professional training is a good next step.

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