Gemara

36a (Rav): If Reuven was guarding a deposit, and he gave it to another Shomer, Reuven is exempt;

(R. Yochanan): He is liable.

(Abaye): Rav exempts because he gives it to one with intellect. R. Yochanan obligates because the owner does not want his object in others' hands.

(Rav Chisda): Rav did not say his law explicitly. Rather, it was inferred from an episode in which gardeners used to deposit their hoes with Leah. One day, they left them with Reuven (one of the gardeners). He heard a wedding, and left them with her. They were stolen. Rav exempted Reuven.

The one who heard this ruling inferred that Rav (normally) exempts a Shomer who gave to another Shomer is exempt. This is wrong! Rather, because they always deposited with her, they showed that they consent for her to guard them.

36b (Rava): The Halachah is, a Shomer who gave to a Shomer is liable, for the owner can say that he trusts the oath of the first Shomer, but not of the second.

42a (Mishnah): If Reuven deposited coins with Shimon, and Shimon gave them to his small children (minors) to guard (and they were lost), he is liable, for this is not the way people guard.

(Rava): One who deposits expects that the Shomer will entrust the deposit with (adult) members of the Shomer's family.

Rishonim

Rif and Rosh (Bava Metzia 3:23): Rava taught that a depositor expects the Shomer to entrust the deposit with members of the Shomer's family.

Rosh: If people of the Shomer's family were negligent, the Shomer is liable. If the Shomer would be exempt, his wife and children would consume the deposit, for also they are exempt (they have no money to pay)!

Hagahos Ashri: If it is known that the Shomer does not guard things himself, rather, he gives them to others, it is as if the owner himself deposited with the one that the Shomer normally gives it to. The first Shomer is totally exempt, even if the latter Shomer was negligent.

Rosh (6): If the owner normally deposits this with the second Shomer, the first Shomer is exempt. The owner cannot say that he does not trust him.

Rambam (Hilchos Nizkei Mamon 4:11): If a Shomer gave to a Shomer, the first Shomer is liable for damage. The owner says 'why did you give it to another, and not guard it yourself?! Pay me, and go to Din with the second Shomer. I trust your oath, but not the second Shomer's oath.' Therefore, if the owner always deposited with the second Shomer, the first is exempt. He tells the owner 'what you deposited with me, you deposited with him Emesh (last night).'

Rambam (Hilchos She'elah 4:8): If a Shomer gave the deposit to his minor children, he is liable, unless the second Shomer proves that he was not negligent.

Mordechai (271): If the owner normally deposited different things with Levi, he can say 'I did not want this in his hands', like Abaye. Rav Chisda taught that even R. Yochanan according to Rava would exempt in the case of the gardeners, but Abaye holds that R. Yochanan would obligate.

Mordechai (272): If Shimon returned Reuven's deposit to Reuven's wife, and she has Da'as and Reuven lets her buy and sell, Shimon swears and he is exempt. Surely Rava exempts, for Reuven trusts her with everything. Even Abaye exempts, like the case of the gardeners. Also, one who deposits expects the Shomer to entrust it with members of the Shomer's family; this is similar.

Maharik (77): The gardeners used to deposit their hoes with Leah. Rav Chisda concluded 'since they used to deposit with her', without mentioning the hoes, for he holds like Rava, therefore, a Shomer would be exempt even for a different deposit. Rav Ashi (who arranged the Gemara) put Rav Chisda after Abaye's teaching to teach that Abaye agrees that we cannot infer Rav's opinion from here (since they used to deposit the same item). Since Reuven lets his wife buy and sell, it is as if he normally deposits everything with her.

Poskim

Shulchan Aruch (CM 291:26): If a Shomer gave to a Shomer, the first Shomer is liable. The owner can say 'I trust your oath, but not the oath of the second Shomer.' Therefore, if the owner always deposits this with the second Shomer, the first Shomer is exempt.

Shach (42): The same applies if it is known that the first Shomer does not guard things himself, rather, he gives them to others.

SMA (46): 'Always' means that he deposits with him most of the time. The Tur gives this exemption in Siman 305 regarding a Shomer Sachar, and does not say 'always'. All the more so, a Shomer Chinam is exempt even if the owner did not always use the second Shomer!

Beis Yosef (DH v'Yesh): The Rambam connotes that he is exempt only if the owner used to deposit this item with the second Shomer. This is unreasonable. If he trusts him with money, it does not matter which property! It seems that the Rambam just cited Gemara. However, this is difficult, for he said twice that he used to deposit this same item! Perhaps he teaches that we exempt only for a deposit no bigger than he normally deposited. This requires investigation.

Beis Yosef (DH v'Heicha): If Yosef used to deposit with Levi, and David gave Yosef's deposit to Levi and Levi was negligent and cannot pay, this is like one who gave to his wife to watch, and she cannot pay (Sefer ha'Terumos 3:3). The Tur discussed this in Siman 72 (David is liable).

Beis Yosef (DH Kosvu Talmidei, citing Talmidei ha'Rashba): The Ramban and Rashba say that if a Shomer Sachar gave to his wife or children or one with whom the owner used to deposit, and the second Shomer swore that he was not negligent and it was stolen or lost, the first Shomer is liable. Even though we are unsure, the second Shomer is exempt, and the first Shomer cannot say 'perhaps it was Ones.' He must clarify it. If the second Shomer does not want to swear or went away, the first Shomer must pay. Rabbeinu exempts even if the owner says that he knows that it was stolen, for the first Shomer says that perhaps it was Ones. This is like one who claims Vadai, and the other is unsure whether he owes to him.

Shach (44): Rabbeinu's opinion is difficult. Even when a Shomer Chinam gave to a Shomer Sachar and the latter wants to swear, the owner can refuse to trust his oath, and the first must pay. All the more so the first must pay when the second does not swear! Perhaps Rabbeinu explains that this is when we do not know where the deposit is, but if we know that it is not in the second Shomer's house, he is believed. This is not primary. The Terumas ha'Deshen obligates a Shomer even if we are unsure if was lost through negligence or Ones, for he must swear clearly. He cannot say 'I do not know.' The Nimukei Yosef in the name of the Ro'oh exempts, but the Rema brings only the Rosh and Tur, who disagree. I say that even if the owner is unsure (and also the Shomer), he is liable. This is unlike one who does not know if Ploni owes him (and also Ploni is unsure). The Nimukei Yosef exempts, like one who does not know whether or not he borrowed. Tosfos equates him to one who does not know whether or not he returned.

Ketzos ha'Choshen (12): Hagahos Ashri says that if the Shomer gave it to one whom the owner normally deposits it with, even if the latter says that it was lost through negligence, the first Shomer is exempt. He can say 'you trusted him; I did not.' This implies that if we knew that he was negligent, the first Shomer would be liable, like the Rosh.

Note: Our text of Hagahos Ashri totally exempts the first Shomer.

Rema (72:30): If Shimon lent to Reuven and took a security, and he gave the security to Levi, and it was lost due to Levi's negligence, and Levi cannot pay, Shimon is liable. Reuven's claim is against him, even if he normally deposits with Levi.

SMA (96): Even though we say that a depositor expects the Shomer (David) to entrust the deposit with (adult) members of David's family, if his wife or child was negligent, David is liable, for he received the deposit. The Rambam holds that the wife or child is liable, but not David. He agrees that when David gave it to a stranger (Levi), David is liable, even if the owner normally deposits with Levi. This is negligence.

Rebuttal (Shach 134): The opposite is more reasonable! Even R. Tam and the Rosh (who obligate one for his family's negligence) agree here. The Yam Shel Shlomo (Bava Kama 1:32) says that if the owner took Levi to Din and could not collect, David is exempt. One is liable for his family's negligence, lest one give the deposit to his household and they will consume it. This is not necessarily true; the Mordechai (Bava Metzia 281) holds that R. Tam obligates also here. Sefer ha'Terumos and the Tur agree. In any case, surely the Rambam and Shulchan Aruch exempt also here. The Beis Yosef, Darchei Moshe, Bach, Mordechai and Agudah say so. Perhaps there is a printing mistake in the SMA, and instead of 'Mechaber' it should say 'Rema'. Even so, I disagree. In Siman 291 the Rema brought two opinions. Here he relies on what he wrote there.