Gemara

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?

Bava Kama 10a (R. Zeira): There is another case in which one responsible for part of the damage is responsible for all of it. If one handed an ox over to five, and one of them (Reuven) was negligent (and left), and it damaged, he is liable.

Question: If it cannot be guarded without Reuven, obviously, he is liable!

Answer: Rather, it could be guarded without Reuven.

Objection: If so, what did he do?!

Rishonim

Rambam (Hilchos Nizkei Mamon 4:7): If one handed his ox over to five, and one of them was negligent and left, and it damaged: if it can be guarded only with five, the one who left is liable. If it can be guarded with those who remained, even those who remained are liable.

Magid Mishneh: This is clear from Bava Kama 10. All the Meforshim explain that even those who remained are liable.

Rebuttal (Even ha'Ezel): Rashi and the Rosh exempt the one who left! It seems that the Rambam explains 'what did he do?!' like Tosfos (DH Mai), to mean he did no more than the others. However, regarding put bundles on an animal, the Rambam (6:14) explains 'what did he doesn't like Rashi, that he is exempt! It seems that the Rambam holds that the words can mean either. Here he obligates, for if a Shomer gave the deposit to another Shomer he is liable if it gets damaged, and similarly he is liable if the deposit does damage. This is why the Rif did not need to bring our Sugya (he already ruled that a Shomer who gave the deposit to another Shomer is liable).

Rosh (Bava Kama 1:9): If the ox could be guarded without Reuven, he did nothing, i.e. he is totally exempt! This is when the others did not cease to guard it and say that they do not want to guard it any more. Since they continued to guard it and can guard it themselves, they are liable and he is exempt. This is because the owner handed his ox over to all of them, whoever can guard it, and they agreed to guard it without their colleague. If they told him 'since you do not want to guard it, also we cease to guard it', they are all liable.

Rosh (Bava Metzia 3:23): One deposits knowing that the Shomer will entrust it with his wife and children (Bava Metzia 42b). This refers to oaths. The wife or children will swear to exempt themselves. The owner cannot say that he does not trust their oath. We do not exempt the Shomer from paying for them if they were negligent. If not, everyone will feed deposits to his wife and children, and they will lack what to pay and also he will be exempt!

Teshuvas Rema (27): When it is known that the Shomer does not guard it himself, he is totally exempt, even if the one to whom he gave it was negligent. Even though a husband is the primary Shomer, he is liable for the negligence of his household only if they lack money to pay themselves. If two Shomrim accepted a deposit, they are Arevim for each other (Bava Kama 10a). The Rambam (Shevuos 38b DH Selik) says that they are like two who borrowed together. It seems that this is when they explicitly stipulated.

Rebuttal (Shach 77:1): The Ramban, Rosh and Ran say that a Stam deposit (with two Shomrim) is like a loan to two borrowers. The proof from Bava Kama is invalid. We tried to explain when one who damaged partially is liable for all the damage. We did not discuss liability like an Arev.

Rosh and Ran (Shevuos 5:2 and 17a): The Yerushalmi says that when two deposited with one, if one of them comes to take his 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 it will be clear from the document.

Question (Ran, ibid.): In Bava Metzia (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 the owner gives half the Kefel when there were two Shomrim and one agreed to pay for his half. If he has full responsibility, this is the same question!

Answer #1 (Ran): Even though letter of the law, each is responsible for the entire amount, perhaps the owner gives half the Kefel in this case, for the partner paid his entire obligation. If he would pay the other half, he could collect it back from his partner.

Question: We asked (96a) whether She'elah b'Ba'alim (one who borrows from his worker is totally exempt) 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 (and Answer #2 to question (i) - Ran and Nimukei Yosef Bava Metzia 19a, 55a): One partner is responsible for the other's share only like an Arev.

Poskim

Shulchan Aruch (CM 396:7): If one handed his ox over to five, and one was negligent and left, and it damaged: if it can be guarded only with five, the one who left is liable. If it can be guarded with those who remained, even those who remained are liable.

SMA (12): It is incumbent on all of them to guard it. He should not have left without their permission.

Shulchan Aruch (ibid.): Some say that this is only if they said 'since you do not want to guard it, also we cease to guard it', but if they did not cease to guard it, he is exempt and they are liable.

SMA (13): He is exempt, for he can say 'I relied on you. The owner handed it over to all of us, to whoever can guard it.'

Gra (15): This is like the Rambam. Also Tosfos (DH Mai) explains (if it can be guarded without him) he did nothing more than the others. They are equally liable. We cannot say that he is exempt, for if Reuven dug 10 Tefachim, Shimon dug another 10, and Levi dug another 10, all are liable (51a).

Shulchan Aruch (ibid.): Some say that this is when the others said 'since you do not want to guard it, also we cease to guard it', but if they did not cease to guard it, they are liable and he is exempt.

Gra (15): This is like the Rosh, Nimukei Yosef and Rashi. Why did the Mechaber (and Rema) bring only this opinion in Siman 383:4 (and 381 and 388:7)?

Machaneh Efrayim (Shomrim 27): The Ran, Nimukei Yosef and Rosh say that if two accept a deposit, each accepts to guard only half, but is an Arev for the other (if the other cannot pay), just like two who borrowed together. The Ramban says that Levi becomes an Arev whenever David acts on Levi's word. However, the Ran holds that an Arev must benefit in order to obligate himself. When a Shomer Chinam takes a deposit, there is no Hana'ah with which he can obligate himself! The Rosh says that a Shomer need not do Meshichah, for the Mafkid relied on the Shomer's word and ceased to guard his object. However, if one says 'deposit with Moshe, and I will be an Arev', this is Asmachta. He assumes that Moshe will guard well. The Yerushalmi must hold like the Rosh, that each is a Shomer on the entire item, or discusses borrowers.