Commentary for Kiddushin 100:12
<big><strong>גמ׳</strong></big> מנהני מילי אמר רמי בר חמא דאמר קרא (ויקרא יח, יח) אשה אל אחותה לא תקח לצרור התורה אמרה בשעה שנעשו צרות זו לזו לא יהא לו ליקוחים אפי' באחת מהם אמר ליה רבא אי הכי היינו דכתיב (ויקרא יח, כט) ונכרתו הנפשות העושות מקרב עמם אי קידושין לא תפסי בה כרת מי מחייב
AND IT ONCE HAPPENED TO FIVE WOMEN, AMONGST WHOM WERE TWO SISTERS, THAT A MAN GATHERED A BASKET OF FIGS, WHICH WAS THEIRS, AND WHICH WAS OF THE SEVENTH YEAR,<span class="x" onmousemove="('comment',' The Talmud discusses this below.');"><sup>21</sup></span> AND DECLARED, BE - HOLD, BE YE ALL BETROTHED UNTO ME WITH THIS BASKET, AND ONE ACCEPTED IT ON BEHALF OF ALL: THE SAGES RULED, THE SISTERS ARE NOT BETROTHED. <big><b>GEMARA: </b></big>Whence do we know it? - Said Rami B'Hama: Because Scripture saith, and thou shalt not take a woman to her sister, to be a rival to her [li-zeror]:<span class="x" onmousemove="('comment',' Lev. XVIII, 18.');"><sup>22</sup></span>
Daf Shevui to Kiddushin
Daf Shevui to Kiddushin
Therefore, he explains that the mishnah follows Rabbah who said that when an act, such as marrying two sisters, cannot be performed consecutively, it can also not be performed simultaneously. If he tries to marry two sisters at the same time, neither are married.