The core sugya begins at Pesachim 79b:11-12, which constructs a double-layered rov/miut problem: the Jewish people are split half-pure and half-impure, but within that impure half the deciding members are women, so that removing the women (who are only reshut on the first Pesach) leaves the impure as a true miut that is doche'd to Pesach Sheni — a minority sustained only by a minority within it. Tosafot on Pesachim 79b:1 sharpens this, asking whether such a mixed group is called an "ish" or a "tzibbur" and why the tehorim are not required to act betumah on behalf of so small a fraction, precisely because there is no rov tmei'im to begin with.
Tzitz Eliezer 5:11 addresses the phrase itself, distinguishing one person's standing relative to two from two people's standing relative to a larger group, and concludes that one against two is treated as a true miut d'miuto — a minority of a minority — which even Rabbi Meir, who is machmir to be concerned for a miut, does not take into account.
The broader principle that a concern for a miut can nonetheless exist is rooted in Yevamot 61b:4-6 and Niddah 31b:22, both invoking Rabbi Meir's position that one is chayesh lemiuta even where the probability is small, with Tosafot on Yevamot 61b:2 qualifying that this concern applies only where the minor could in principle still reach the feared state, not where that possibility is foreclosed.
Ketubot 15a:5 and Sanhedrin 79a:8-9 show the Gemara combining two majorities (rov ha'ir together with rov sia) or treating a kavua as mechetza al mechetza, illustrating how the sugya handles compounded probabilities of majority and minority rather than one bare rov alone.
Chullin 11b-12a limits when a miut is disregarded at all, distinguishing cases where it is efshar livdok (possible to examine) from cases where it is not, and uses Rabbi Meir's chashash lemiuta regarding a possible perforation to test the boundaries of when the majority is followed despite a known minority.
Practical halachic codes apply these rov/miut mechanics to mundane cases: Mishneh Torah, Forbidden Foods 3:23-24 permits a whole barrel of brine based on a sampled kosher fish, Mishneh Torah, Ritual Slaughter 11:11 records that most places do not check the lung for a possible nekev absent a specific concern, and Shulchan Arukh, Yoreh De'ah 110:4 discusses meat bought from a majority-kosher set of shops being batel berov mishum sefek sefeka, with the gloss disputing whether a prohibition still attached at its source can ever be treated as a true majority.
Sdei Chemed, Maarechet Alef 382 and Sdei Chemed, Maarechet Alef 301 discuss the exegetical rule that a miut after a miut comes to expand rather than to further restrict, a related but distinct application of double-minority language to drashah methodology rather than to a statistical concern for a minority of people or cases.
Several further Yevamot and Gittin passages touch the same rov/miut machinery in testimony and status cases: Yevamot 117b:8-10 discusses following rov dei'ot even among disqualified witnesses, Yevamot 117a:10-12 and Yevamot 25a:2-3 weigh a kala that does or does not pasek, Yevamot 122a:3-4 relies on an single gentile's report against a group, and Gittin 89b:1-2 and Gittin 64b:12-13 likewise turn on whether a minority possibility (a minor posing as an adult, a maidservant not yet developed) is enough to unsettle an otherwise clear majority, while Eruvin 80b:6 sets a numeric threshold for merubin versus mu'atim in an unrelated eruv context.
Rav Samson Raphael Hirsch frames acharei rabim lehatot as a broad halachic principle by which the minority's distinct character is treated as nullified into the majority across many areas of halacha, both where the majority is directly visible and where it is assumed as a matter of general practice.
A few of the retrieved sources sit outside this cluster and do not address a minority-within-a-minority as such: Vayikra 13:40-43 discusses baldness and tzaraat, Rashi on Yehoshua 4:18 discusses the ark carrying its bearers, and Siftei Kohen on Shulchan Arukh, Choshen Mishpat 39:2 and Beit Yosef, Choshen Mishpat 388:4 discuss shibud and mesira disputes rather than compounded rov/miut reasoning.
Among the later poskim, Tzitz Eliezer 18:13 takes up a practical version of the compounded-minority question, rejecting an attempt to combine two separate minority risks into a majority that would warrant Shabbat desecration, and concluding that recovered elderly patients may not be driven home from the hospital by a Jewish driver on Shabbat or Yom Tov, though a non-Jewish driver may be considered.