The older rooster will need time to regain his mojo. Just as the presence of a dominant rooster results in lower testosterone level in subordinates, I think that a beatdown/deposing does the same thing to a once dominant rooster. Sometimes they never regain their confidence.
Young roosters are always looking to depose the head roosters. It's all about the DNA. I would remove the remaining young rooster from the flock, reintroduce the senior rooster, wait a couple of weeks before reintroducing the young rooster. Expect fighting, but hopefully the old rooster can...