Commentary for Kiddushin 102:16
ת"ש שנים שקדשו שתי אחיות זה אינו יודע איזו קידש וזה אינו יודע איזו קידש זה נותן שני גטין וזה נותן שני גטין ה"נ שהוכרו ולבסוף נתערבו דיקא נמי דקתני אינו יודע ולא קתני אין ידוע שמע מינה
yet if they forestall [the Rabbis ruling] and marry them, they are not compelled to divorce them,<span class="x" onmousemove="('comment',' Lit., 'they are not taken out of their hands'.');"><sup>21</sup></span> [Thus:] only halizah and then yibum [is permissible], but not yibum and then halizah, because he may infringe [the interdict against] the sister of one bound to him by the Levirate tie.<span class="x" onmousemove="('comment',' Lit., 'he comes into contact with the sister etc'. - Thus: A betrothed X or Y, who are sisters, but does not remember which. On A's death, his brothers B and C perform halizah and yibum with X and Y respectively. Now, when B performs halizah with X, C may marry (perform yibum) Y. For if A had betrothed Y, she is C's yebamah, whom he must marry; while if A had betrothed X, Y is a stranger to C, and he may certainly marry her. For though Y is then the sister of X, who was bound to him by the Levirate tie, and such is forbidden, that tie has already been dissolved by the halizah which B performed. But before the tie is dissolved by halizah marriage is forbidden; hence only that order is permissible, viz., halizah by one brother first and then yibum by the second, (Of course, that is only permissive: the second too may perform halizah, if he does not wish to marry her.) The prohibition mentioned in this note is only Rabbinical, and therefore not insisted upon if the brothers marry both sisters without consulting a Rabbi previously, Yeb. 23b,');"><sup>22</sup></span>