Yevamot 28A

לכתחלה קתני. ולימא ליה: גזירה דלמא קדים וחליץ לראשונה ברישא! ״ולא מתייבמות״ קתני, דליכא דין יבום הכא כלל.

implies that the Mishnah teaches a case in which ḥalitza is performed ab initio. This indicates that this is the first course of action and the only way to resolve the situation. The Talmud asks further: And let him say to him the following: The Mishnah prohibits levirate marriage ab initio in this case, due to a rabbinic decree lest he proceed and perform the ḥalitza with the first sister first, by which it would be prohibited to consummate the levirate marriage with the second. Perhaps for this reason the rabbis decreed that it is prohibited to consummate the levirate marriage even if ḥalitza was performed with the second sister. The Talmud answers: It teaches: They may not enter into levirate marriage. This indicates that the halakha of levirate marriage does not apply here at all. Accordingly, even after the fact, if he performed ḥalitza with the second sister, the halakha of levirate marriage would not apply to the first sister.

ולימא ליה: גזירה שמא ימות, ואסור לבטל מצות יבמין! רבי יוחנן למיתה לא חייש.

The Talmud asks: And let him say to him a different reason for the ruling of the Mishnah: It is due to a rabbinic decree lest the second brother die, and it is prohibited to negate the mitzva of levirate marriage. Perhaps for this reason it would be forbidden to consummate the levirate marriage in the case where two sisters happened before him for levirate marriage, and not due to the prohibition proscribing the sister of a woman with whom he has a levirate bond. The Talmud answers: R' Yoḥanan was not concerned over the possibility of the death of a brother, and in his opinion there is no need to make a decree to address such cases.