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Adding a second dog to a situation will change the dynamics. A dog may be very obedient to you when it is just you and them but when you add another dog to the mix, they sometimes feel that they need to show that they are the boss to the other dog and then they become less obedient.
If you get a puppy that shouldn't be that much of a factor as long as you don't get another male. A female would be a better choice as you won't have dominance issues when she matures into an adult. You will want to keep his obedience and other training up to keep him obedient.
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Border collies usually do really well with companions. They often form exceptional bonds with one another and even can be trained to work as a team. Often they will team up if put into a situation where it might be helpful to work as a team. I don't think adding a female will cause a problem at all. I think you will likely even find that she learns much quicker having him around. You do have to be sure you keep his exercise levels pretty much the same to keep him stimulated or he might take it out on the other dog. It is common to focus more on a new pup and training them which could lead to problems if he becomes bored, but since you do work him, that shouldn't be much of a problem as long as you keep that in mind.
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