Let me first clarify the concept of the superlative before answering your question.
The superlative is the highest extent or degree of something. In English, to express which of more than two items has the highest degree of the quality expressed by the adjective, we append the suffix “-est” to certain short adjectives (like “newest” for “new”) or put the adverb “most” before longer adjectives (like “most reliable” for “reliable”). Superlatives can be either objective or subjective in character.
Objective superlatives are those that express comparisons of facts or conditions as perceived without distortion by personal feelings, prejudices, or interpretations. The comparative measure is factual or numerically demonstrable, so the validity of the superlative statement can be accepted without question, as in these statements: “The 2.16-km San Juanico Bridge between the islands of Samar and Leyte is the longest bridge in the Philippines.” “Jupiter, with a mass two and a half times that of all the other planets combined, is the biggest planet in our solar system.”
Let’s see what happens when we apply the objective superlative to more than one in a group of things: “The 2.16-km San Juanico Bridge between the islands of Samar and Leyte and the 1.098-km Buntun Bridge over the Cagayan River between Cagayan Province and Apayao Province are the two longest bridges in the Philippines.” “Jupiter and Saturn are the two biggest planets in our solar system.”
In the two sentences above, is the strength of the objective superlative diminished by applying it to more than one in the group of things being compared? Not at all. Clearly, there’s no grammatical or logical impediment to applying the superlative to more than one of the comparables.
Now let’s consider subjective superlatives, which are those that express comparisons as conditioned by a personal mindset or state. They are value judgments—opinions—and it would be difficult to determine or dispute their accuracy, truthfulness, or reliability. These two sentences that you presented are superlatives of that kind: “This is one of the most beautiful girls in our class.” “They are two of the most respected elderly men in our neighborhood.”
Is the strength of a subjective superlative in the two sentences above diminished by applying it to more than person in the group? Again, the answer is clearly no. In fact, we can apply the superlative in such sentences to even more people and neither the strength nor validity of the superlative would be diminished: “They are five of the most beautiful girls in our class.” “They are the ten most respected elderly men in our neighborhood.”
Undoubtedly then, whether objective or subjective, superlative attributes can be applied to any number of things being compared without detracting from the strength of the comparison and the correctness of its grammar and semantics.
