Yevamot 96B

מתני׳ בן תשע שנים ויום אחד שבא על יבמתו, ואחר כך בא עליה אחיו שהוא בן תשע שנים ויום אחד ⁠— פוסל על ידו. רבי שמעון אומר: לא פסל. בן תשע שנים ויום אחד שבא על יבמתו, ואחר כך בא על צרתה ⁠— פסל על ידי עצמו. רבי שמעון אומר: לא פסל.

Mishnah: If a boy aged 9 years and one day had sex with his yevama, and afterward his brother, who is also 9 years and one day old, had sex with her, the second brother disqualifies her to the first one. R' Shimon says he does not disqualify her. If a minor aged 9 years and one day had sex with his yevama, and afterward that same boy had sex with her rival wife, he thereby disqualifies her to himself, and both women are now forbidden to him. R' Shimon says he does not disqualify her.

גמ׳ תניא, אמר להם רבי שמעון לחכמים: אם ביאה ראשונה ביאה ⁠— ביאה שנייה אינה ביאה. ואם ביאה ראשונה אינה ביאה ⁠— ביאה שנייה נמי אינה ביאה.

Talmud: It is taught in a baraita that R' Shimon said to the Rabbis: If the first sexual act of a 9-year-old is considered a proper act of sex, then the second act is not an act of consequence, just as the sex of one adult yavam after that of another adult yavam is of no effect. And if you say that the first sexual act is not considered a sexual act, the second act of himself or his brother is also not a sexual act. However, the Rabbis maintain that as the sex of a 9-year-old is like a levirate betrothal, one sexual act can take effect after another.

מתניתין דלא כבן עזאי. דתניא, בן עזאי אומר: יש מאמר אחר מאמר בשני יבמין ויבמה אחת.

The Talmud comments that according to this explanation, the Mishnah is not in accordance with the opinion of ben Azzai. As it is taught in a baraita that ben Azzai says: There is levirate betrothal after levirate betrothal in a case of two yevamin and one yevama. In other words, if they both performed levirate betrothal with her, their actions are effective and she is forbidden to them both. The reason is that she has ties to each of the two men, which means that each levirate betrothal is effective in forbidding the other man.