Reference for Yevamot 3:9
שְׁלֹשָׁה אַחִין נְשׂוּאִין שָׁלֹשׁ נָכְרִיוֹת, וּמֵת אַחַד מֵהֶן, וְעָשָׂה בָהּ הַשֵּׁנִי מַאֲמָר, וָמֵת, הֲרֵי אֵלּוּ חוֹלְצוֹת וְלֹא מִתְיַבְּמוֹת, שֶׁנֶאֱמַר (דברים כה), וּמֵת אַחַד מֵהֶם יְבָמָהּ יָבֹא עָלֶיהָ, שֶׁעָלֶיהָ זִקַּת יָבָם אֶחָד, וְלֹא שֶׁעָלֶיהָ זִקַּת שְׁנֵי יְבָמִין. רַבִּי שִׁמְעוֹן אוֹמֵר, מְיַבֵּם לְאֵיזוֹ שֶׁיִּרְצֶה, וְחוֹלֵץ לַשְּׁנִיָּה. שְׁנֵי אַחִין נְשׂוּאִין לִשְׁתֵּי אֲחָיוֹת, וּמֵת אַחַד מֵהֶן, וְאַחַר כָּךְ מֵתָה אִשְׁתּוֹ שֶׁל שֵׁנִי, הֲרֵי זוֹ אֲסוּרָה עָלָיו עוֹלָמִית, הוֹאִיל וְנֶאֶסְרָה עָלָיו שָׁעָה אֶחָת:
If there were three brothers married to three strangers, and the wife of one of them died, and the second made a ma'amar in her and died, they receive chalitzah and are not taken in yibum, it being written (Deuteronomy 25:5): "…and one of them dies … then her yavam (singular) shall come upon her" — when the linkage of one yavam, and not two, is upon her. [For so long as he (the one who made the ma'amar) did not marry her, the linkage of the first is still upon him, and there is added to it the linkage of the second by way of the ma'amar; and when he dies, there remains upon her the linkage of two yavmin.] R. Shimon says: He may take either one in yibum [He holds that we are in doubt as to whether ma'amar acquires completely or does not acquire at all. Therefore, he can take either one in yibum. For if ma'amar acquires, she has only the linkage of the second upon her; and if it does not acquire, she has only the linkage of the first.], and he gives chalitzah to the other. [For one is not exempt by the (sexual) taking of the other (in marriage). For it may be that ma'amar does not acquire, in which instance there are "two yevamoth coming from two houses." And he cannot take both in yibum, for it may be that ma'amar does acquire, in which instance there are two yevamoth coming from one house." The halachah is not in accordance with R. Shimon. And even though we say in our Mishnah that according to Scripture there is (such a thing as) linkage of two yavmin, as indicated by: "Her yavam (singular) shall come upon her," in the gemara it is shown that it obtains only by rabbinical ordinance, lest it be said that two yevamoth coming from one house are taken in yibum.] If there were two brothers married to two sisters and one of them died, and then the wife of the second one died, she is forbidden to him forever, since she was forbidden to him at one time. (See 3:7).