Yevamot 27B

בעלת הגט ובעלת מאמר, איזו מהן קודמת? בעלת הגט עדיפא, משום דאתחיל בה בחליצה, או דלמא: בעלת מאמר עדיפא, משום דקרובה לביאה.

If two women happened before a single yavam for levirate marriage, and one is a woman who received a bill of divorce and the other is a woman who received levirate betrothal, which has precedence for ḥalitza? Is the woman who received a bill of divorce preferred because he began the process of ḥalitza with her, as presenting a bill of divorce represents the first step separating the woman from him? Or perhaps the woman who received levirate betrothal is preferred, because she is closest to being able to enter into permitted sex. The act of levirate betrothal is generally done just prior to levirate marriage and is equivalent to the act of betrothal in non-levirate contexts. As a result, levirate betrothal strengthens the connection between the woman and the yavam. For this reason, it may be preferable to perform ḥalitza with the woman who received levirate betrothal.

אמר רב אשי, תא שמע: ומודה רבן גמליאל שיש גט אחר מאמר, ומאמר אחר גט.

Rav Ashi said: Come and hear: The rabbis disputed the ruling with regard to a yavam who performed levirate betrothal with one sister-in-law and then performed it with her rival wife as well, or conversely, gave both women a bill of divorce. In such cases, would the second levirate betrothal or bill of divorce be effective? It was taught: And Rabban Gamliel concedes that a bill of divorce is effective after levirate betrothal. Therefore, the bill of divorce that the yavam gave to one yevama after having performed levirate betrothal with the other yevama is effective to some degree. Similarly, Rabban Gamliel concedes that levirate betrothal performed after a bill of divorce is effective.