I think some mix of the above responses will help answer this question.
It really depends on which 2 pits you have.
I fostered a young female pit for a while with my resident, then 3yo female pit. There was entirely too much competition between them and in the interest of my resident girl, I had to return the foster to her owners (who decided to keep her, thankfully).
My original dog is socialized out the wazoo, gets along with *every* dog, but gets along with them all differently. I’ve dog sat many dogs here and observed different levels of chemistry.
I finally brought home an approximately 7 year old male pit, and they instantaneously got along better than any two dogs I’ve ever seen. Laurelai, my female, is like kid sister, endlessly playful and annoying, also very touchy feely and cuddly. Percy, the newcomer has just the right level of playfulness to keep her happy and can step it up to hold his own, but there’s no competition, he lets her climb all over him with the patience of a saint. He’s literally the perfect companion for her. As a result, my life is not more stressful by having two dogs, it’s just twice as fun.
In your situation though, you are considering a puppy, which is automatically more work, but if your dog gets along with puppies and you can be there to supervise their interactions, AND I would suggest picking a puppy of opposite gender only because your dog already shows tendencies toward competitiveness and a small submissive female is less likely to create drama than a male who when he turns 2 or 3 decides it’s time to challenge your existing dog.
I should also say that Percy shows some mild dominance and intense interest in other dogs that he sees on the street, but not allowing him to focus on other dogs this way shows him that it’s not acceptable to attempt to challenge other dogs, or be that interested in them, because wherever we are is my territory and not his. And once he meets them he’s fine, but if you can get break your dog’s focus on other dogs by not allowing him to actively patrol the window or organizing supervised play dates with dogs he gets along with, you can gradually expand his socialization and circle of friends. If your dog is truly dog aggressive, he might be happiest as an only dog for a while, but if he’s only aggressive to strange dogs, you might be okay introducing a carefully selected friend for him. Grabbing some books on the issue will help prepare you for what’s to come as well. Good luck!