If you want to get a new snake, and If you were willing to risk it and keep them together, you would want to buy another cage anyway to quarentine the new snake. Even if you buy from a responsible breeder, it still could be sick. It is also possible that you could quarentine the new snake, find nothing wrong, but it could be carrying an illnesss that would be more dangerous to your ball python.
I'm not saying that it is impossible to keep two different snakes together, but I would not suggest it.