Commentary for Kiddushin 103:1
האחד חולץ לשתיהן והשנים אחד חולץ ואחד מיבם אם קדמו וכנסו אין מוציאין אותן מידם
the one [brother] must perform halizah with both, and of the two, one must perform halizah [first] and the other yibum; yet if they forestall [the Rabbis' ruling] and marry, they are not compelled to divorce them. Thus, only halizah and then yibum, but not yibum and then halizah, because he may infringe [the interdict against] a yebamah's marriage to a stranger.<span class="x" onmousemove="('comment',' Lit., 'a yebamah to the market place'. - The general reasoning is the same as in the previous case. When the one brother frees both sisters by halizah, the others may perform halizah and yibum. But before the one brother has performed his task, one of the sisters may be his yebamah, and so neither of the other two brothers can perform yibum.');"><sup>1</sup></span>
Daf Shevui to Kiddushin
If Reuven and George (non-brothers) die, and each has only one brother (Shimon and Bill), Shimon must perform halitzah with both women and Bill must perform halitzah with both women. Neither can have yibbum with either woman because each woman might be the sister of the woman with whom he is truly liable to have yibbum, the z’kukah (because we don’t know who betrothed whom).
If Reuven has two brothers, Shimon and Levi and George still only has Bill as a brother, Bill must have halitzah with both Rachel and Leah. With regard to Shimon and Levi, one brother must have halitzah with both women but the other brother can have yibbum. This rule was explained in the above mishnah quoted in the previous section. The second brother can have yibbum with either women because if she was truly the woman who Reuven betrothed, then yibbum is proper. If Reuven betrothed the other sister, then the sister with whom this brother now has yibbum is no longer the sister of his z’kukah, because this other sister has already received halitzah from his other brother.
If both brothers preemptivelymarry both sisters, they are not forced to have a divorce. Certainly the second marriage is okay, and even the first marriage was only problematic in the beginning, before the second sister had yibbum. Even though the woman whom he married may have once been the sister of his z’kukah, after she has had yibbum the other sister is no longer a z’kukah.
With regard to the last example, the Talmud emphasizes that one brother must first performe halitzah and then the other can perform yibbum. But if one brother performs yibbum first he may be marrying someone else’s yevamah, and until someone performs halitzah with her, she may not be remarried.