Who knows two?
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Two are the wives of Lemech, and Hevel according to one medrash (Bereishis Rabbah 22:3).
The gemara in Bava Kamma 29b states:
2 things that aren't "technically" in a person's property but (in terms of halachic responsibilities) the Torah makes it as if it is in a person's property:
A dream dreamt twice is a sign that it will be fulfilled (Brachos 55b).
2- are the instances where the Torah accepts the testimony of just one witness (see: Rambam, Hilchos Eidus 5:2):
a) by a Sotah- so that she does not drink the bitter waters
b) by an eglah arufah- to prevent its neck from being broken
(d'rabbanan we also accept one witness who's testifying that a woman's husband died)
2 is the number of first Orthodox female rabbis.