Reference: Elsewhere (Pirkei d'R. Eliezer 36, and Bereishis Rabah Parshas Vayeshev) we say that a twin sister was born with each of the Shevatim.

Observation: Below (124a), we establish this like Rebbi, for according to Rabanan, a Bechor does not receive a double portion in Shevach that the property increased after the father died.

Implied question: If a father left to a Bechor and a Pashut a slave or Tamei animal, he [or it] serves [the Pashut] for one day and [the Bechor] for two days (above, 13a. It seems that all agree to this!)

Answer: Indeed, [Rabanan agree that] if the Bechor said "divide, or I will take a double portion in Shevach", he may.

Question: Why didn't we discuss when the orphans rented out [the animal, and even so Rebbi holds that the Bechor gets a double portion]?

Answer: Now it teaches a bigger Chidush. Even though it was Muchkeres (another was caring for it, and the father shares the profit) or rented in the father's lifetime, the cow is not considered Ra'uy, and the Bechor gets double [according to Rebbi, even in the rental - Maharam].

Observation: Everywhere, we consider Makirei Kehunah to be Muchzak;

In Gitin (30a), it says that if Ploni lent money to a Kohen or Levi [and stipulated that Ploni will keep the Terumah or Ma'aser that he separates from his Peros, and deduct from the loan... we ask that the Kohen or Levi never received the tithes, and answered that we discuss Makirei Kehunah or Leviyah. It is as if he already received them].

Explanation: This is because it is a small gift, and it is forbidden to retract, and even through mere words [one may not retract his promise. (Since the Yisrael only has Tovas Hana'ah, i.e. rights to pick to whom to give it, it is like a small gift.]

Implied question: [Why is the Kohen Muchzak? The Yisrael] can retract if he wants!

Answer: Even so, as long as he did not retract, [the Kohen] is like one who is Muchzak.

Limitation: This is only if the animal was slaughtered in the father's lifetime. If not, no (the Bechor does not receive two portions). Since [the father] died, who acquires?!

Question: Below, we establish this like Rebbi, and in Kidushin (58a) he holds that it is as if they were not separated!

Answer #1: The Tana here holds like Rebbi only regarding that a Bechor receives two portions in Shevach.

Answer #2: Only regarding Matanos of Terumah and Ma'aser, Rebbi holds that that it is as if they were not separated. However, Matanos here, of the foreleg, jaw [and stomach], which are recognized (the owner need not decide what to give), he holds that it is as if they were separated.