One always must remember with cockerels, "Today's behavior is NOT an indication of tomorrow's behavior." They might stay friends... and they might become sworn enemies. You can't make that prediction based on the current behavior - but you can enjoy it while it lasts!
I've kept all-cockerel flocks on occasion, some for years. Not only will they pal around like Besties, but they play like a bunch of frat-boys. If I had the wherewithal, I'd always have a "Bachelor Flock." Not only are they highly entertaining, but the eye candy is divine!
In my experience if cockerels are of similar size, the same general age and more docile types, it can work. But even then once any chink in the armor becomes evident, things can go sideways in a hurry.
Each individual will have its own traits. One thing many people on this forum enjoy is that each chicken has its own personality, male or female. While we expect certain behaviors as typical or normal, you do not get guarantees, anything can possibly happen and often does.
Is it possible for cockerels to become besties? I've seen that, I do not consider that uncommon, even after the pullets have started laying so fairly well into maturity. Is it possible for mature roosters for be besties? Yes, possible but less likely. By "besties" I mean they hang out together instead of forming separate flocks. Typically pullets or hens also hang with them, like a normal flock, but occasionally they pair off.
Typically they know which one is dominant though they may occasionally have skirmishes to reinforce that. Sometimes only the dominant one can mate when they are together, sometimes either may mate a pullet or hen. With chickens you do not get guarantees on behaviors.
As others have mentioned, the relationship can go extremely violent at any time. But that is not inevitable. You just never know.
Yes, my number one and two roosters are half brothers and share the majority of my hens. Blue is definitely in charge and Ron is subordinate. They had mild skirmishes the first year to establish dominance, but now hangout together peacefully. Ron will occasionally irritate Blue and earn a chasing, but two minutes later they can be side by side tidbiting the girls over something that one of them found.