It is a common misconception that littermates will always get along. Littermates, especially those of the same sex, probably argue as often as human siblings argue. Dogs that are spayed and neutered will have much greater chances of getting along with each other, since hormones won't be involved. Another common misconception is that it is a great idea to get two puppies at once. Two young puppies can actually be harder to train because it will be harder to get each to focus on you.
Young puppies can learn many valuable lessons from older, more settled dogs.
Like humans, it will be important for each dog to have some space. Each dog should have its own crate and should be fed there. Like people, some dogs just don't like each other for whatever reasons. There are some chemistries that just don't blend well together. Just because your dog doesn't like one dog, doesn't necessarily mean he won't like all dogs.