Gemara

(Beraisa): "V'Chichesh Bah" excludes one who admits to one of the brothers or partners who claimed from him.

Question: What is that case?

If he admitted that he owed half to one partner, he should be liable for denying the other half!

Answer: Rather, they say that they lent to him together, and he admits to borrowing the entire amount from one partner.

Bava Metzia 33b (Mishnah): If Reuven deposited with Shimon an animal or Kelim, and they were stolen or lost, Shimon could swear and be exempt. If he paid and did not want to swear, and the thief was found, he pays Kefel (double) to Shimon.

34b - Question: If Shimon paid half, what is the law? (Does Reuven give to Shimon rights to collect half the Kefel?)

Question: If two partners borrowed, and one of them paid, what is the law?

96a - Question: If two partners borrowed from Shimon, and he was working for one of them, is this She'elah b'Ba'alim?

Perhaps the owner must be working for all the borrowers. Or, perhaps the partner who is employing the owner is exempt for his half?

Rishonim

Rif and Rosh (Shevuos 16b and 5:2): The Yerushalmi expounds "he denied his fellowman" to exclude one who lied to partners. R. Yosi inferred that if two borrowed from one, they are Arevim (guarantors) with responsibility for each other, even if they did not write this.

Rosh and Ran (17a): (The Yerushalmi continues) if two deposited with one, and one comes to take his half, we do not heed him. He will deny his half, and he will be liable. If one deposited with two, and one of them denied it, he is liable. If the other denied it, he is liable. If he deposited a Perutah with them, the Shevu'ah takes effect on a half-Perutah. Here, the Shevu'ah is on a Perutah. The Ramban explains that the verse refers to one who exempted himself through the Shevu'ah, and he would have been liable had he admitted. We exclude one who accepted a deposit from partners, and one partner claimed it, for had he admitted he would not be obligated or permitted to give it to him until his partner came. Therefore, he is not liable if the Shevu'ah was false. R. Yosi inferred that if two borrowed from one, even though they wrote only 'Ploni and Almoni borrowed 100 from Levi', each is liable for the full amount. When two deposited with one, one depositor cannot say 'you are obligated to return my share.' The Shomer says 'I am liable only to both of you (together).' Likewise, when two borrowed from one, he can say 'I lent to both of you, and neither of you is exempt by paying just your share. Rather, each of you is responsible for the full amount.' The custom is not to rely on this. Rather, they write that they are responsible and Arevim for each other, so the document will make it clear. Since one is exempt when he swears to one partner, we infer that even if the Shomer admits, he need not give to him. When one deposits with two, they are liable, for each is responsible for the full amount. The Yerushalmi asked that each swears about half a Perutah. It answered that each is liable for the full amount.

Rosh and Ran: The Rif brought the Yerushalmi briefly. R. Chananel rules like it. R. Efrayim rejected it, for the Bavli did not say that one who denies to one partner is exempt. It exempted only one who says that he borrowed the entire amount from one partner. This is wrong. The laws of both Beraisos are true. The Bavli expounds 'to exclude one who admitted', therefore it teaches exemption due to admission. The Yerushalmi expounds 'to exclude one who denies', therefore it teaches exemption of one who totally denies.

Rosh: Some ask from a Tosefta (Bava Metzia 1:21) which says that if two borrowed in a document and Ploni paid his part, the document is still valid, and the lender writes 'I received your share.' This implies that Ploni is not responsible for the rest. This is not difficult. The case is, the document explicitly says that each of them borrowed 100. The Ra'avad explains that the Yerushalmi exempts one who denies to one partner, for the other partner can claim everything. Just like when two lent, one can claim everything, when two borrowed, the lender can claim everything from either. This is not the correct Perush of the Yerushalmi. Also, if one partner claimed the entire amount, and the borrower swore to deny it, the other partner can claim only his share. Most of this is from the Ramban.

Rambam (Hilchos Malveh 25:9): If two borrowed in one document, or bought one purchase, or if one partner borrowed or bought for the partnership, they are Arevim for each other, even though they did not specify.

Rosh (Bava Metzia 3:3): Rashi said that we ask about a partner who paid half, which is all that is incumbent on him. Rashi does not mean that he is exempt if the other partner will not pay or swear. The partners are Arevim for each other! Rather, he paid what is incumbent on him, and perhaps his partner will pay (therefore, perhaps the owner immediately gives the Kefel to him).

Question (Ran, ibid.): We asked whether an owner gives to a Shomer half the Kefel when the Shomer agreed to pay for half. Then we asked whether he gives half the Kefel when there were two Shomrim and one agreed to pay for his half. If he is responsible for the entire animal, this is the same question!

Answer (Ran): Even though letter of the law, each Shomer is responsible for the entire amount, perhaps the owner gives half the Kefel when Shimon paid his entire obligation. If he would pay the other half, he could collect it back from the other Shomer. Alternatively, one Shomer is responsible for the other's share only like an Arev. The owner can claim from him only if the other Shomer has no property. We asked (96a) whether She'elah b'Ba'alim applies when partners borrowed and the owner was working for one of them. If a partner is fully responsible, this is like working for all the borrowers! Rather, one partner does not intrinsically owe the entire amount. He is only an Arev for his partner's share.

Poskim

Shulchan Aruch (CM 77:1): If two people borrowed together or bought an item together (Rema - or accepted a deposit together, whether with or without a document), they are Arevim for each other, even though they did not specify.

Beis Yosef (DH v'Da): The Yerushalmi does not distinguish whether or not there was a document. The Rambam mentioned a document, for this is the usual case. Alternatively, he teaches that it is only if they borrowed like one.

Rebuttal (Shach 2): The Yerushalmi says 'even if they did not write responsibility for each other.' This connotes that there was a document! The Rambam and all Poskim mention a document. It is difficult to say that the same applies without a document. The Rashba (1082) clearly distinguishes. Perhaps the Beis Yosef and Rema mean that the same applies without a document when it is clear that they borrowed in partnership.

Defense (Gra 4,5): The Yerushalmi and Bavli establish the Beraisos to discuss even a loan with witnesses. The Yerushalmi cannot discuss a loan document, for then either partner could claim by himself!

SMA (2): It is reasonable that the lender intended that if one cannot pay, he will collect from the other.

Gra (2): (Money owed for) a sale is like a loan. It is included in the verse "and he denied' (Bava Kama 105b).

Rema (Teshuvah 27): If two overseers were appointed, this is unlike two who borrowed together. Borrowers get Hana'ah, so they are Arevim for each other. The lender lent with this intent. Why should one overseer accept to be an Arev for the other?