I also think that it's better if you get the two dogs before the first dog hits one year old because the older a dog is the more set in their ways they get.
If the two dogs are far apart in age they aren't going to be going through the same growing stages as one another. Just like a 10 year old person and a 20 year old person (you can't throw the two together and expect them to be interested in the same things).
That's why its always important to take your dog to see your new puppy a few times at the breeders to make sure their personalities are compatible.
What is the age difference between your two? and have they been introduced yet?
Don't let all this stuff psych you out though, just make sure your ready for whatever happens.
I'm sure they'll be fine
