So the young ones are about 18 weeks old. They are almost mature but not quite. Immature chickens are automatically lower in the pecking order than mature chickens. It does not matter if they are male or female. If a chicken lower in the pecking order invades their private space, especially around a feeder or on the roost, the dominant one will maybe peck the weaker one. Not always but often. So the immature chickens tend to avoid the mature flock. It is less painful to them that way. Chickens that grow up together tend to hang together anyway, at least until they are really mature. Sometimes they keep separate flocks within a flock for years unless something seriously disrupts the pecking order in both groups.
It is highly possible that the older chickens (usually a hen but it can be the rooster) are not keeping the young ones away. Sometimes it does happen, but it is more likely that the younger ones choose to stay away. It does not matter if there is a rooster in that group or not.
A dominant rooster in a flock has certain duties to perform in taking care fo his flock. He finds them food and protects them. Another rooster in the flock also has certain duties but he does not have the overall responsibility of the dominant one. Plus if the subordinate rooster gets out of line, the dominant one will kick his butt. A rooster can go through a severe personality change if he becomes the dominant one.
In my opinion, if the older rooster is taking care of his girls and is behaving himself around people, I'd keep him. You don't know how the younger one will change if he becomes dominant. He may be fine, but then he may not. And regardless which one you get rid of, the younger group will probably not really hang with the older any more than they do now.
