For two Mitzvot of the same breed, as opposed to two disparate Mitzvot: Pesachim 102b
Using the bread of a meal which extends the Shabbat Travel Border [Techum] for one of the two loaves of the Shabbat Meal: Shabbat 117b
Having one Kohen give two women the "Sotah water" to drink at once: Sotah 8a
Having one Kohen purify two lepers at once: Sotah 8a
Having one person pierce the ears of two avadim at once: Sotah 8a
Performing the "Eglah Arufah" rite on two calves at once: Sotah 8a