Related for Yevamot 37:15
ועוד תניא ר"ש פוטר בשתיהן מן החליצה ומן הייבום שנא' (ויקרא יח, יח) ואשה אל אחותה לא תקח לצרור בשעה שנעשו צרות זו לזו לא יהא לך ליקוחין אפי' באחת מהן
from the <i>halizah</i> and from the levirate marriage, for it is said in the Scriptures, And thou shalt not take a woman to her sister, to he a rival to her,<span class="x" onmousemove="('comment',' Lev. XVIII, 18. ');"><sup>40</sup></span> when they become rivals to one another<span class="x" onmousemove="('comment',' As in the case cited, where each of the two brothers was married to one of each pair, and when the first brother died all his widows became subject to levirate marriage with the second brother and thus become rivals. ');"><sup>41</sup></span> you may not marry even one of them!<span class="x" onmousemove="('comment',' Even the first widow. Consequently R. Simeon's exemption applies to all, which shews that he recognizes no distinction on the question of the levirate bond between one levir and two levirs! ');"><sup>42</sup></span>