Genèse 41:50

And to Joseph were born two sons before the year of famine arose, which Asenath who had been brought up in the house of Potipliera prince of Tanis bare to him.

בטרם תבא שנת הרעב, seeing that Yaakov arrived in Egypt at the beginning of the second year of the famine and he died at the end of seventeen years of having lived in Egypt, he said to Joseph prior to his death: “the two sons of yours who were born before I came i.e. Ephrayim and Menashe, will be to me as Reuven and Shimon.” Any children of Joseph, and he had such, who were born to him after his father arrived in Egypt would be considered as regular grandchildren of Yaakov in respect of the eventual parceling up to the tribes of the soil of the Holy Land. (48,5-6).

וליוסף יולד, the singular mode used by the Torah in describing the birth of these two sons of Joseph suggests that they were twins. At least they were presumably born during the same night.

שנת הרעב, the first year of the famine; the Torah adds: בטרם תבא, for they were born during the last year of the 7 years of plenty. Joseph was 37 years old when they were born. They were therefore 19 years old when Yaakov died. My teachers (Taanit 11) explained that the wording “before the onset of the famine,” is meant to teach that a person must not engage in marital intercourse during years of famine.

אשר ילדה לו אסנת, this is to teach that Joseph did not have any additional wives, i.e. both of his sons were born by the same mother.

בטרם תבוא שנת הרעב, “before the year of famine arrived.” The Talmud in tractate Taanit folio 11 teaches that from this verse we learn that during a famine man and his wife are not to cohabit [so as not to add more hungry mouths at that time. Ed.] In light of this it is difficult to understand how Levi could have sired Yocheved during those years as we have been told that she was born when her mother entered Egypt on her way there with Yaakov’s family. (Compare Genesis 45,6) Rabbi Yehudah hachassid, opined that the prohibition of cohabiting with one’s wife during periods of famine is valid only for a prophet who has been told by G–d directly that there was going to be such a famine. Joseph was such a prophet who had been inspired by G–d to advise Pharaoh of future events. Levi who did not have any direct knowledge concerning the origin of this famine, and who had caused it, was therefore not obliged to abstain from the commandment to be fruitful and to multiply. According to a different interpretation, the brothers Levi and Joseph took different sides in an argument aired in the Talmud, tractate Yevamot folio 61 concerning that if the commandment to be fruitful has already been fulfilled if one has fathered two sons. A second opinion holds that until one has sired both a son and a daughter, one cannot claim to have fulfilled the commandment. Since Levi at this time had not yet sired a daughter, he had felt entitled to maintain normal family relations with his wife. Joseph had already sired two sons, and he held that thereby he had fulfilled the commandment.