I'd try 'em both. My own two young buff cockerals, both by the same dad but likely out of different hens, are in with only five of their own half or full sisters and it's still going just fine and I would have missed out on a lot of amusement already if I'd gotten rid of one or separated them back when they were at their most obnoxious. One's clearly dominant, the other submissive, and they seem to have formed quite the alliance in looking after their shared little flock. Their most recent amusing stunt was deciding to put the other young flock cockerel in the yard, one of their own clutch-mates and an olive egger, finally and firmly on notice by teaming up to chase him around both the back and front yards. Unfortunately, the buffs' version of running as fast they could was just a slow jog for the olive egger (I named him Raptor Boy for good reason) and it was quite the chase to behold, the olive egger trotting along just ahead of these two determined waddling balls of feathers who were following him wherever he went, looking back over his shoulders and not knowing WHAT to think. Poor Raptor Boy finally got so spooked that he flew up on the hood of my car, where he crouched staring down at his tormentors, the feathers on just the nape of his neck sticking straight out with fright and bewilderment. I finally rescued him, put him back in the backyard with his own pullet flock, and I guess the buffs were satisfied with what they'd done or were maybe just too tired to continue because they haven't bugged him since.
Roosters are usually viewed by most, I think, as only being able to socially exist as the one solitary guardian of a flock or perhaps living with other bachelor roosters in a bachelor flock. The more I see of their behaviour, though, the more I'm beginning to think of them as being more like male lions. For a long time it was thought that it was always one male lion per pride and no more. Now we know that it's a bit more complex and that some prides have multiple males, usually brothers, who form alliances and who are content to share their pride in order to better protect them against other competing males. I think some roosters are like that. Some get along just fine, the usual expectations be damned. But you'll never know if you've got a pair like that unless you give them a chance to show their stuff.
That said, I do still have a spare pen which I was planning to use if the two buff cockerals I have DIDN'T get along. But they do. So far. So the buff flock has both pens to use right now and seem to enjoy doing so. As long as you're prepared in a similar way should things go south, I say try the multiple route. You might see some interesting behaviours you'd never expect.