Keeping two puppies together has everything to do with the care and socialization of those puppies and nothing to do with the breed of the puppies.
Aggression toward other dogs are as common in the Pit Bull breed as with any other breed according to The Real Pit Bull, a website dedicated to researching and dispelling myths regarding the breed. Therefore it is no more of an issue to keep two Pit Bull puppies together than it would be to keep two Golden retriever puppies together.
Importance of Socialization
As with any breed of dog, if the American Pit Bull terrier is raised correctly and socialized at an early age, it will have no more likelihood of instigating confrontations with other dogs than any other breed of dog would.
Aggression is not the norm for Pit Bull terriers. The Pit Bull terrier genetically has the temperament of a goofy, stable and friendly breed. In fact since the breed is so people-friendly and eager to please they do make poor guard dogs contrary to popular belief and the conceived notions due to media reports.
If a Cocker spaniel is abused, beaten, neglected or trained to fight they are just as at risk of becoming fearful of humans and aggressive toward other dogs as the Pit Bull is. It just so happens that the people who fight, abuse and neglect dogs usually choose the Pit Bull terrier as their victim because of the undeserved stigma that goes along with the breed.
Safely keeping two puppies together of any breed has nothing to do with the breed but with the care and socialization those puppies receive.