Pitbulls, being a fighting breed are best to only have together when you can supervise. I would never leave them alone together (or with any dog) just to be safe. Depending on breeding they can and will fight till one is dead. I speak from plenty of experience, not just a casual observer. You can pm if you have specific questions.
I've had my dogs alone all day long and they have yet to eat my cat or killed each other.
