This is really just my opinion. There is no law that says you can't keep ball pythons together.
BUT. Every breeder I know keeps all of their snakes housed separately. This is for the benefit of the snakes. A snake is more likely to thrive living by itself then it is with cage-mates. As I said before, keeping two snakes together can double vet bills and you also don't know who is pooping/peeing/shedding. Not to mention the high probability that one of the snakes will stop eating or grow more slowly than the other.