You will notice we have different opinions on this. We all have different conditions and different experiences so that's not surprising. Plus, no one can give you any guarantees as to how living animals will behave. Each one is an individual. But sometimes we can tell you what we think will likely happen.
I also believe there is no magic ratio of hens to roosters. No number of hens will stop two roosters from fighting over the girls if they are going to fight over the girls. They will fight over 20 hens as fast as they will fight over one. Some people have problems with over-mating and barebacked hens with over 20 hens per rooster, let alone just a few hens. There are several different factors involved, not limited to individual personalities, how much room you have, age, and how you manage them. Sometimes it works great, sometimes it is a disaster. We all get different results.
I don't know how much room yo have or how you are set up. What I'd expect to happen if you try introducing a new grown rooster to your flock that already has a resident mature rooster is that the two will fight. They will determine which is boss. That could be a fight to the death or it could become a lot more running away and chasing than fighting. Often it's somewhere in between the extremes. Having enough room so they can run away and get away could be really important. Sometimes one will die or be seriously injured in these fights, sometimes they reach an accommodation as to how they will work together to protect the flock. A typical solution is that each has its own territory out of sight of the other and they each get their own harem. But sometimes they reach other solutions.
Typically with mature adult hens and roosters over-mating isn't a big problem. You can get chickens that never grow up and act like adults but most do. Most of the stories about hens being over-mated or torn up by a rooster are really about flocks with immature pullets and cockerels. But, as I said, some never grow up. I do think that more room seems to be of benefit in this too. But it is something you need to watch.
What are your goals for those boys? Why do you want two roosters with your flock? The only reason you need a rooster is for fertile eggs. Anything else is personal preference. Nothing wrong with personal preference, I have a few of those myself, but that is a choice not a need. I generally suggest you keep as few roosters as you can and still meet your goals. That's not because you are guaranteed problems with more roosters but that problems are more likely. You hatch eggs so you need at least one.
Telling us more about your set-up and how you are managing them might help. Also, what are your goals, why do you want a second rooster with the hens? We might be able to offer some suggestions on how to meet your goals if we know what you have to work with. There usually are solutions.
You can try adding one of the boys with that flock and see what happens. But I'd suggest being there to observe and being prepared to separate two fighting roosters if the need arises.