Gemara

(Rav): If brothers divided, and a creditor (of their father) took the portion of one brother, the division is void.

(Shmuel): That brother lost. He is not compensated.

(Rav Asi): That brother gets a quarter of what was taken, in land and (or) money.

Rishonim

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.

Kesef Mishneh: The Tur says that the Rambam holds that one may collect the entire amount from either, even if both have property. We must say that the Tur had a mistaken text of the Rambam. The Rambam says so only in Halachah 10, when they were Arevim for another's loan. There is different. Since the borrower cannot pay, and the lender collects from an Arev, it makes no difference from which Arev he collects. When two borrowed, each borrows half and is an Arev on the other half. One collects from an Arev only if he cannot collect from the borrower.

Rambam (10): If there are two Arevim for a loan, when the lender comes to collect, he may collect from whichever he wants. If one did not have the entire debt, he collects the rest from the other.

Rebuttal (Ra'avad): This is not clear. Rather, we follow the custom. We learn from Nochrim to Yisrael.

Magid Mishneh: Bava Basra 107a supports the Rambam. A creditor may collect from any brother, even though we hold that a person's property is an Arev for him. Some (Ramban 107a) strain to reject the proof, and say that this is when one of the brothers has no Beinonis, or one brother's land was an explicit Apotiki (the loan is collected only from it). The Tosefta (Bava Basra 11:8) says that if there were two Arevim on a loan, the lender may not collect from one of them, unless they specified that he may. The Ramban says that the same applies to two who borrowed together. The Rashba (Bava Metzi'a 34b) holds like the Rambam (we follow the Gemara in Bava Basra against the Tosefta). There is no source for the Ra'avad. Also, what will be done in a place where there is no custom?

Rosh (Shevu'os 5:2): When two borrowed, Avi ha'Ezri says that one is liable for the other's share due to Arvus. The lender may claim Ploni's share from Almoni only if he cannot collect from Ploni. The Ramban allows claiming it in any case. This is the Halachah in practice.

Ran (Shevu'os 17a) and Nimukei Yosef (Bava Metzi'a 19a): A Tosefta (Bava Basra 11:8) teaches that when there are two Arevim, l'Chatchilah one may collect only half from each, unless they specified that he may collect the full amount from either. Two who borrowed together are like two Arevim. However, the Rambam allows collecting the full amount from either Arev.

Nimukei Yosef (ibid.): The Rambam explains the Tosefta to say that one may not collect from either Arev before claiming from the borrower, unless he stipulated. The Rashba says that two Arevim are unlike two borrowers. An Arev is like a security. Shibud is not half-way. When partners borrow, each uses half for himself (so his primary responsibility is only for half). Perhaps the same applies to one who deposited with two Shomrim.

Question #1: In Bava Metzi'a 34b, we asked whether an owner gives to a Shomer rights to collect half the Kefel (if the thief will be found) when the Shomer agreed to pay for half the deposit. Then we asked whether an owner gives half the Kefel when there were two Shomrim and one agreed to pay for his half. If he is responsible for the entire deposit, this is the same question!

Question #2: 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!

Answer to both questions (Ran and Nimukei Yosef ibid., and 55a): One partner is responsible for the other's share only like an Arev. The owner can claim from him only if the other partner has no property.

Hagahos Maimoniyos (Mishpatim Teshuvah 63): If both Arevim have money, one may not claim everything from one. R. Noson and Chachamim (Bava Kama 53a) argue (about an ox that pushed an animal into a pit) about getting paid from one what the other should pay, only when one cannot get paid from each what it should pay.

Poskim

Shulchan Aruch (CM 77:1): If two people borrowed together or bought an item together they are Arevim for each other. If one lacks property, everything may be collected from the other. If he has property, one may not collect from the Arev first (Rema - unless they specified that they are Arevim Kablanim, and he may claim from whichever he wants).

Beis Yosef (DH u'Mah she'Chosav b'Shem): The Magid Mishneh says that the Ramban holds like the Rambam, unlike the Rosh (who says that the Ramban permits collecting the entire amount from either l'Chatchilah). Many Meforshim hold like the Rambam, so we follow him.

Gra (9): The Yerushalmi says that borrower are Achara'in (responsible) for each other. Sefer ha'Terumos says that this connotes Kablanos (an Arev from whom one may collect before claiming from the borrower). This is wrong. It also connotes regular Arvus.

R. Akiva Eiger: Even though the document need not say that they are Arevim for each other, if it says so we do not say that this is extra to teach that they are Kablanim.

Shulchan Aruch (132:3): If there are two Arevim for a loan, the lender may collect from whichever he wants. If one did not have the entire debt, he collects the rest from the other. Some disagree and say that he collects half from each. However, if one does not have, he may collect everything from the other.

SMA (77:11): Chachamim did not make the lender toil to collect half from each.

SMA (77:3): An Arev for another's loan is in place of the borrower. The Shibud does not take effect half-way.