Who knows two?
Please cite/link your sources, if possible. In about a day, I will:
Upvote all interesting answers.
Accept the best answer.
Go on to the next number.
According to one Ladino version of Echad Mi yodea (there is more than one version) "Two are Moshe and Aharon" which resolves the seeming duplication of "Two are the Tablets" and "Ten are the Commandments"
2 is the minimum number for a plural word that is darshoned (expounded) in the Talmud.
2 is the number of Pesach Sedarim for those who live outside of Israel.
Two are the Chief Rabbis of Israel.
And, as as a result of the settlement brokered by Rabbi Isacc Elchanan Spektor in 1835, two were the Rosh Yeshivas of Volozhin.
Two (ט"ו) equals 15. (Suggested by my 9-year-old son.)
סימני כשרות לדגים - Signs that a fish is Kosher
Two are the wives of Lemech, and Hevel according to one medrash (Bereishis Rabbah 22:3).
Shnei Luchos Habris
סימני כשרות לבהמות - Signs that an animal is Kosher
מעלה גרה - chews cud
מפרסת פרסה - split hooves
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.