Gemara

(Mishnah - R. Meir): If Reuven's wife gave birth for the first time and had two boys (we do not know which was born first), if the father died and the children are alive, if they paid (from the estate) before dividing it, they paid (the Kohen keeps the money);

If they divided the estate before paying it, they are exempt.

Suggestion: The father died within 30 days.

Question: Surely, after dividing they are exempt because each can say "I am exempt (I am not the Bechor)";

Also before dividing they can say this! (The father was never obligated.)

(R. Yirmeyah): If two people in a city, both named Yosef ben Shimon, bought a field together, a creditor can collect from (up to half of) it (even though he cannot prove which of them borrowed);

He tells each "if you owe me, I collect your debt. If not, your partner owes me. I take his share."

Objection (Rava): One's property is like an Arev (guarantor) for his debts. If one has no claim against the borrower, he cannot collect from the Arev!

48b (R. Yirmeyah): If one Yosef ben Shimon bought a field from the other, a creditor can collect from it;

He tells him "if you owe me, I collect your debt. If not, the land was Meshu'abad (had a lien on it) to me before you bought it."

Objection (Rava): If one has no claim against the borrower, he cannot collect from the Arev!

Rishonim

Tur (CM 49, b'Sof): If one was an Arev for two Yosef (ben Shimon)'s, whether they borrowed from one lender or from two, just like the lender(s) cannot claim from Yosef, he (or they) cannot claim from the Arev.

Poskim

Shulchan Aruch (CM 49:7): If one Yosef borrowed from Reuven, and one of them bought a field from the other Yosef, or they were partners in a field, Reuven cannot seize the field based on the following claim: 'if I lent to you, surely I may collect from your field. If I lent to your namesake, since he has no Bnei Chorin (land that he still owns), you bought land on which I have a lien.'

Gra (32): Sefer ha'Terumah learned this from the following case of partners.

Question (Tumim 21): The Rema (129:8, brought below) says that if the borrower denies the loan, and the Arev admits that he borrowed, the Arev must pay. Here, the lender has a document. Yosef denies it. I.e. he says that the other Yosef borrowed, for witnesses signed that one of them borrowed. The other Yosef denies borrowing. If so, this Yosef must pay, for his property (that he bought from the other Yosef) is like an Arev! Further, it is clear from Sa'if 7 that if others testify that this Yosef borrowed, this helps for a Milveh Al Peh. Admission is like 100 witnesses! We must say that they are like other witnesses testifying about the loan, so it is like a Milveh Al Peh, which cannot be taken from Meshu'abadim (property that the borrower sold). Therefore, we do not say 'in any case, he owes him.' This applies to one who bought from the other Yosef, but if he inherited from him, we hold that a Milveh Al Peh is collected from heirs! If Levi denies that he borrowed, and his son Shimon admits that Levi owes, his admission does not obligate Levi, but after Levi dies, his admission obligates himself! Likewise, if Yosef says that he did not borrow, this means that the Yosef he inherited from borrowed, so Reuven collects! The same applies if one Yosef was an Arev for the other. Therefore, the answer I gave above is wrong. There is no source for this law. (Rather, the lender collects from the field.)

Rebuttal (Nesivos ha'Mishpat 21): The document is not a mere Milveh Al Peh. If the witnesses will come and testify, the lender collects from Meshu'abadim. Other (witnesses) do not help like I said above (in a case when he knows about the loan only through the document - see 19 below). When the buyer himself admits that the document is valid, there is a lien on the property. Also, the Gemara did not give the Tumim's reason. The Gemara compares it to the case of two males. The Torah wrote this debt, so the Tumim's reason does not apply. If there are different reasons for the cases, why does the Gemara equate them? The Ketzos ha'Choshen answered that the document is not Muchach mi'Tocho. (By itself, it is not a proof, so it is invalid.) This is wrong. The Mechaber (Sa'if 7) does not require a document to be Muchach mi'Tocho, just the Shach (11) brought this opinion. The Mechaber connotes that he discusses a lender and borrower. Rather, the case is that the lender's claim is based only on the document. The document does not say which Yosef borrowed. If two witnesses say that one Yosef borrowed on Nisan 1, and two others testify that one Yosef did not borrow that entire day, we do not join them to obligate the other Yosef. This is like Chetzi Davar (the testimony of either pair by itself is worthless), which is invalid (30:13). Similarly, when one Yosef says 'I did not borrow', even though it follows that the other one borrowed, we do not join this to the witnesses (on the document), since that was Chetzi Davar. Do not say that it is full testimony about the Arev. Since one can claim from the Arev only after claiming from the borrower, and their testimony does not obligate the borrower, it does not obligate the Arev. This is unlike an Arev who admits. That is like 100 witnesses. Just like Beis Din cannot obligate the borrower through these witnesses (who say that the other Yosef did not borrow), they cannot obligate the Arev. When two males were born, Beis Din can testify that there is a lien on the property, but still we do not collect, for Beis Din does not obligate payment based on what they know. The same applies to an Arev for two Yosef's. Since the document and witnesses do not obligate the borrower, it is as if they do not exist, even to obligate the property. All the more so, if one Yosef inherited from the other or was an Arev for the other, he is exempt. Perhaps the loan was paid, and he left the document with the lender because it cannot be used to collect. We hold that one can claim 'I paid' against a document that cannot be used to collect from Meshu'abadim.

Shulchan Aruch (ibid.): Similarly, if they are partners in the field, he cannot say 'in any case I may collect half.'

Shach (19): The same applies if one Yosef inherited a field from his uncle Yosef. (The creditor cannot tell him 'even if I did not lend to you, rather, to your uncle, I may collect from the field.') I proved so in Takfo Kohen.

Rebuttal (Tumim): What I said above refutes the Shach. (Yosef admits that he inherited a field that Reuven has rights to collect from it, so he must pay.)

Ketzos ha'Choshen (9): The Shach is astounding. Surely, even regarding a Milveh Al Peh (a loan without a document), if it is known that the Morish (who bequeathed property) owed, the heir must pay. Perhaps we can defend the Shach based on Bechoros 48b. R. Yirmeyah derived that if one Yosef bought a field from the other, the creditor can collect from it in any case. Rava rejected this, for one collects from property only because it is like an Arev. Seemingly, since it is not clear who is the borrower, the document is meaningless, even if we know through witnesses which one borrowed. Such a document of acquisition is invalid. Since witnesses need not see the money given, and it is not know who authorized this document, what was R. Yirmeyah's reason to say that the lender collects? We must say that he held that even if it is not Muchach from the document who borrowed, in any case there is a lien on the field, since the lien is clear from the document. This is whether one Yosef bought it from the other, or they are partners in it. Rava refuted this. Since the document is unclear, there is no claim against the borrower, so the Arev is exempt. If so, likewise an Arev for Yosef is not obligated, since the document did not obligate the borrower. If the Arev admits that the borrower borrowed, surely he is liable even if the borrower denies. Here, it is known only through the document, so letter of the law the borrower is exempt, since the document is not Muchach mi'Tocho. Sa'if 7 connotes that there is a Milveh Al Peh, i.e. when they testified about the loan.

Nesivos ha'Mishpat (18): If one Yosef inherited a field from the other, this is like when he bought from the other, i.e. if he knows about the loan only through the document. If he admits that the other Yosef borrowed and did not pay, he is liable.

Shulchan Aruch (ibid.): The lien on a borrower's property to pay his debt is due to the law of an Arev. In Arev is obligated only when one may claim from the borrower himself. Here, Reuven cannot claim from the borrower himself, so he cannot collect from his property. It follows that if one was an Arev for two Yosef's, whether they borrowed from one lender or from two, just like the lender(s) cannot claim from Yosef, he (or they) cannot claim from the Arev.

Be'er ha'Golah (30'): The case is, at the time of the loan it was not clarified whether he lends to each by himself, or if both loans were to one of them.

Shach (20): It seems that the same applies to a Yosef who was an Arev for the other Yosef. Just like the lender cannot claim from the borrower, he cannot claim from the Arev.

Question (Ketzos ha'Choshen 9): This is astounding. Even without the document, since Yosef (whom the lender claims from) claims 'I did not borrow', he admits that the other Yosef borrowed and now denies it. An Arev must pay in such a case (129:8)! The Tur exempts an Arev for two Yosef's because he does not know to whom the lender lent. Since the borrower is exempt, also the Arev is exempt. If the Arev would know that this Yosef borrowed, even if Yosef denies it, the Arev is liable. The Shach (129:19) says that this is only when the borrower is a Nochri. He connotes that if the borrower is a Yisrael, the Arev is exempt. There is no source for this. Whenever the borrower denies, whether he is a Nochri or Yisrael, the Arev is liable.

Nesivos ha'Mishpat (19): Also here, Yosef is exempt if he knows about the loan only through the document, for the document is worthless. If he admits that the other Yosef borrowed, he is liable.

Nesivos ha'Mishpat (21): The Tumim argued with the Shach. Why did the Tumim agree to the Mechaber's law that an Arev for both Yosef's is exempt? The Arev admits that the borrower borrowed, and now he denies the loan! The Ketzos answered that the Arev does not know which one borrowed. This is difficult. Both deny, so in any case the Arev must pay!

Rema (129:8): If the borrower says 'I paid', and it was a Milveh Al Peh, he swears Heses and also the Arev is exempt. This is even if the borrower is a Nochri. However, if the borrower says 'I never borrower', the Arev must pay.