when i moved in with my boyfriend, it put our dog total to 3 males, 2 intact, one not.
thats 2 bully male bullybreeds(mine neutered, his not) and my toy breed(intact as well). My 2 males are reliable together, and can be "loose" together, but by that, i mean, they can be loose in the same general area as i am. in reality, they would be fine with each other if i left the room/house, with the way they scavenge garbage, etc, im not sure the house would be fine....
my boyfriends male however does not get thrown into this. He is actually extremely social with other dogs, but he is so "in your face",and rough, he has no off switch, and charges into everything, for the small dog, its just plain scary, and for my bully, it could trigger him to react.
all 3 of the males can walk together, they can even be in a large open field off leash together(we go to the dog park when there are no other dogs there), and can and do go to OB and psa training classes together, sit beside eachother quite nicely in class, etc.
we don't keep them separate because they DO fight, we keep them separate because we don't want things to escalate into the makings of a fight about to happen. Big difference, but a key one, i think.
IMO, there is every chance in the world your male rotties could get along quite well. BUT, in order to find that out, you would have to put your dogs in a position to see what would happen, and what could happen is a fight.
Could you break it up?? sure, maybe. but the fight would have already happened, and the damage(both physically, and to your relationship with your 2 dogs, as well as their view of other dogs) would already have been done.
to me, its not worth it to "lets see what they do", especially not with dogs that large, its not an issue with the breed, i love rotties, its an issue with dogs that are just plain big and heavy, trying to pull 2 dogs apart that weigh that much is't something i think i could do alone.
i've done the whole "dogs loose in the house as a pack" nonsense. for most of my dogs, it worked reasonably well, i still crated when i wasn't home, i still crated at night, etc. But i had too many "oh, i guess these two don't get along, NOW i will crate and rotate these 2" to know that the damage gets done fast, and for me, its not worth it anymore.
Either way, if you DO get this additional dog, no matter HOW friendly they seem towards eachother, i would STILL crate and rotate at first, possibly for the first month or longer, don't jump the gun because they seem to get along.
after the initial having them ignore each other phase, i personally would then(this will take 2 people) work with on-leash OB with both dogs in the same training area/yard/park, then walks together, etc etc.
it is better to take it unreasonably slow needlessly, than to try and fix an issue, because the dogs were introduced before it was time.