Short answer. Both of them, solidarily. Article 927 says that where two or more heirs take possession of the estate, they are solidarily liable for the loss or destruction of a thing left to a legatee, even if only one of them was negligent. The legatee may recover the full value from either heir.
What the law says
If two or more heirs take possession of the estate, they shall be solidarily liable for the loss or destruction of a thing devised or bequeathed, even though only one of them should have been negligent.
Civil Code, Article 927 — Solidary Liability of Heirs in Possession. Read the full provision →
What Article 927 provides
Article 927 provides that if two or more heirs take possession of the estate, they shall be solidarily liable for the loss or destruction of a thing devised or bequeathed, even though only one of them should have been negligent. When heirs take the estate into their hands, the things in it that were meant for legatees are in their custody until delivered. If one of those things is lost or destroyed through fault, the article makes all the heirs in possession answerable together — and makes their liability solidary, not merely shared.
What 'solidary' means for the legatee
Solidary liability is the strong kind. It means the legatee may demand the entire value of the lost thing from any one of the heirs in possession, not just a proportional slice from each. He need not sue everyone, or work out who did what; he can look to whichever heir is easiest to recover from for the whole amount. This is a deliberate protection for the legatee, who had no control over the property and no way to prevent the loss. The risk of the heirs' custody is placed on the heirs, and the legatee is spared the burden of apportioning blame among them.
Why the blameless heir is caught too
The striking part is that liability falls on all the possessing heirs even though only one of them should have been negligent. An heir who personally did nothing wrong is still answerable to the legatee for a loss caused by his co-heir. The reason is that they took possession together and together assumed responsibility for what was in the estate; as against the legatee, they stand as one. This does not mean the innocent heir bears the loss in the end. It means he pays the legatee first and settles internally afterwards — the legatee's remedy comes before the heirs' quarrel about fault.
The innocent heir's recourse against the negligent one
An heir who pays for damage he did not cause is not left without a remedy; he is simply put second in line. Having satisfied the legatee, he can seek reimbursement from the co-heir whose negligence actually caused the loss, because as between the two of them the fault, and the ultimate cost, belong to the careless one. So the order is: the legatee recovers in full from any possessing heir, and that heir then recovers from the negligent one. If you are the blameless heir, keep evidence of who handled the property and how the loss happened, since that is what your claim over will turn on.