I have raised unrelated boys together and they were bonded to each other very closely and we never had any trouble. I have also raised girls together with no problems. All of mine have been spayed and neutered though and I think that helps. That way the boys won't want to mark and the girls stay sweet I think.
Yah, it's cute to have a boy and a girl but it doesn't have to be that way. Personally what I believe it all boils down to is their temperment and what works for you and your family. Some dogs will accept any new dog with no trouble and others won't accept any. Socialize your baby young and you'll have a better chance of adding a new puppy when you are ready. Throw out the preconceived notions and find a puppy that gets along well with your puppy and your family...that's what works best
