Your question is an excellent question, and it is discussed by the Si'ach Yitzchak at length. He adds that in fact, from the Beraisa on top of 28b as well as Tosfos Menachos 100a (citing Rashi), it seems as though it is indeed one person who says both "He'ir Pnei ha'Mizrach" and "Ad shebe'Chevron." We may add that the Girsa of Rabeinu Chananel and the Ritva here (citing Rashi, see also Dikdukei Sofrim) is that according to the Iy Ba'is Eima of the Gemara, the person on the roof indeed makes both statements, and the person on the ground answers just "Hen." If so, why didn't the first answer of the Gemara suggest that the person on the ground said both statements, and the person on the roof answered "Hen," as you asked?

(a) The Si'ach Yitzchak answers that the person on the ground is not sure whether the East is bright yet or not. He is asking a question, not making a statement. Since he does not know if the East is bright at all, it is not be appropriate for him to ask "Is the East bright until Chevron ." He should first ask if the East is bright, and when told that it is, he may ask further if the brightness reaches Chevron, since they are two questions (or he may simply ask, "Is Chevron bright?"). Saying "Is the East bright until Chevron" makes it sound as though he knows that the East is bright, and he only wants to know if the brightness reaches Chevron.

Thus, the person on the ground could not have said both statements. However, the person on the roof, could indeed have said both statements. Since he was saying them as a certainty and not as a question, it would be interpreted as meaning, "Not only is it bright in the East, it is even bright until Chevron."

Now the Gemara is clear. The Gemara is asking that if the person on the roof made both statements, what point was there for the person on the ground to say "Hen?" And if the one on the ground was asking, he could not have made both statements, so the one on the roof must have been the one who responded "'Ad shebe'Chevron."

According to the Si'ach Yitzchak, it is not clear why we cannot prove from the Beraisa on top of 28b that the person on the roof says both statements, since it seems clear from the Beraisa that one person said the entire phrase "He'ir Pnei ha'Mizrach Ad shebe'Chevron." As explained above, we can accept that one person said it all only if he was the one on the roof .

The answer could be that if Masya ben Shmuel, who made the statement, was on the ground, he was replying to a "Ro'eh" on the roof who had already said either "Barka'i" or "He'ir ha'Mizrach." Masya, on the ground, was now asking, "Did you mean "He'ir Pnei ha'Mizrach Ad shebe'Chevron "? Although he only added the two words "Ad shebe'Chevron," he repeated the original phrase, "He'ir ha'Mizrach," as well. Now the words of the Beraisa, and even the Mishnah, are more clear. This will also explain the words of Rashi cited by Tosfos in Menachos (who said that Masya said the entire phrase) as well.

(b) According to our Girsa, even the person on the roof does not say the entire phrase, so the answer of the Si'ach Yitzchak will not suffice. I would like to suggest the following answer.

The Gemara maintains that it would not be proper for one person to say both "He'ir Kol ha'Mizrach" and "Ad shebe'Chevron," since the word "Kol" already includes Chevron! Kol means the entire Mizrach, and Chevron is included. Thus, it must be that one person says "He'ir Kol ha'Mizrach," and another person says "Ad shebe'Chevron."

You may be wondering why it is necessary for another person to ask, or to clarify, "Ad shebe'Chevron" if it has already been said that "He'ir Kol ha'Mizrach"? The answer is that it is not necessary. It is only mentioned to allude to the Zechus Avos, as Rashi says. However, a single person who wanted to allude to the Zechus of the Avos would have to leave out the word "Kol" and say "He'ir ha'Mizrach Ad shebe'Chevron," so as not to make a nonsensical or inconsistent statement. The word "Kol" proves to us that there are two people involved.

What about the Beraisa on the top of the page? The Girsa of the Dikdukei Sofrim there is "He'ir Mizrach Ad shebe'Chevron," without the word "Kol," and this may have been the Girsa of the Rishonim. That Beraisa leaves out the word Kol, and indeed maintains that a single person made both statements.

Best wishes,

M. Kornfeld