Short answer. The majority prevails. Where two or more partners manage without divided duties, each may act alone, but once one opposes another's act, the decision of the majority controls. If the managing partners are evenly split, the tie is broken by the partners who own the controlling interest in the partnership.
What the law says
each one may separately execute all acts of administration, but if any of them should oppose the acts of the others, the decision of the majority shall prevail. In case of a tie, the matter shall be decided by the partners owning the controlling interest.
Civil Code, Article 1801 — Two or More Managers. Read the full provision →
Each manager may act until someone objects
Article 1801 governs a partnership where two or more partners are entrusted with management without their duties being carved up and without a rule that none may act without the others' consent. In that setup, each one may separately execute all acts of administration. Day to day, any managing partner can bind the firm on ordinary matters acting alone. The friction you describe arises only when one manager opposes what another is doing. Until an objection is raised, a single manager's act stands; the article's tie-breaking machinery is triggered by the disagreement itself.
The majority decides
Once a managing partner opposes another's act, the article resolves the clash by numbers: the decision of the majority shall prevail. The majority here means the majority of the managing partners, counted by head. So if there are several managers and they divide, the larger group's decision governs, and the opposed act is measured against that. This keeps a single dissenting manager from paralyzing the business, while ensuring that a contested decision reflects more than one manager's unilateral will before it binds the partnership.
Breaking a tie
With exactly two managing partners who disagree, there is no majority to be had — the vote is one to one. Article 1801 supplies the answer for that deadlock: in case of a tie, the matter shall be decided by the partners owning the controlling interest. The stalemate is broken not by seniority or by who acted first, but by capital. The partners holding the controlling interest in the partnership cast the deciding voice. So between two evenly opposed managers, ownership of the firm ultimately settles which choice the partnership follows.
The limits of this rule
Article 1801 applies to acts of administration under a management arrangement with no specified division of duties and no unanimity clause. If your partnership agreement instead assigned separate spheres to each manager, or required consent of all before acting, those stipulations control and this default does not. Acts that go beyond ordinary administration — fundamental changes to the partnership itself — are governed by different rules that may demand unanimity. Within its scope, though, the article gives a clear order: act freely, yield to the majority when opposed, and break ties by controlling interest.