Short answer. Solidarily liable, as a rule. When two or more officious managers take charge together, each can be held for the full amount of any damage — unless you stepped in specifically to save the property from imminent danger, in which case the liability is only joint.
What the law says
The responsibility of two or more officious managers shall be solidary, unless the management was assumed to save the thing or business from imminent danger.
Civil Code, Article 2146 — Delegation by the Gestor. Read the full provision →
The general rule: solidary liability
Article 2146 of the Civil Code provides that the responsibility of two or more officious managers shall be solidary, unless the management was assumed to save the thing or business from imminent danger. When you and a co-manager stepped in to handle your neighbour's affairs without being asked, you both took on the role of officious managers (or gestors). If your management caused damage or loss, the neighbour — or anyone affected — can demand the full amount from either of you alone, without having to go after both proportionally.
What solidary means in practice
Solidary liability means the creditor has a choice. The neighbour can sue you, your co-manager, or both — and collect the full amount from whichever of you has the means to pay. You cannot reduce your exposure by pointing out that the other person was equally involved. If you pay in full, you then have a separate right to seek reimbursement from your co-manager for his share, but that is an internal matter between the two of you. The neighbour's right to collect from either of you in full is unaffected by whatever arrangement you have between yourselves.
The exception: saving from imminent danger
There is a meaningful exception. If you both stepped in specifically because the property or business was facing imminent danger — a fire threatening the building, an urgent repair to prevent flooding, a business obligation about to lapse — and your management was an emergency response to that threat, the liability is not solidary. In that scenario the law is more forgiving: each manager answers only for his proportionate share. The reasoning is that emergency intervention serves the owner's urgent interest and deserves less severe treatment than ordinary unsolicited management.
Which situation applies to you
Whether the exception applies turns on the facts. If you and your co-manager saw a genuine emergency and acted to protect the property from concrete, pressing harm, you may be able to argue that the solidary rule does not apply. If, however, you simply decided to manage the affairs because the neighbour was away or unavailable — without any urgent threat — the default solidary rule likely governs. The distinction between ordinary absence and imminent danger matters significantly, both for how much each of you is exposed and for how a claim against you would be structured in court.