Commentary for Kiddushin 128:11

According to this logic, a father prefers to call his daughters “elder.” So if we had learned only the first clause, we might have thought that R. Meir disagrees with R. Yose only in this case and says that eldest could refer to any except the youngest. But if he says “youngest” we might have thought that he agrees with R. Yose and that this term can only refer to the youngest. Therefore, we need the second clause.

If we did not have the first clause I might have thought that R. Yose disagrees only in this case. Youngest can only refer to the youngest daughter of all. But in the first case, I might have thought that he agrees with R. Meir, that “eldest” could refer to any except the very youngest.
Therefore, we need both disputes to let you know that they dispute in both cases.