Short answer. Yes, each can act alone. Under Article 1801, where two or more partners are entrusted with management without their duties specified and without a rule requiring joint consent, each may separately perform all acts of administration. But if one opposes another's act, the decision of the majority prevails; a tie is broken by the partners owning the controlling interest.
What the law says
each one may separately execute all acts of administration
Civil Code, Article 1801 — Two or More Managers. Read the full provision →
Each manager may act alone
When a partnership names several managing partners but does not divide their duties or require them to act together, Article 1801 gives each of them full run of ordinary management. Each one may separately execute all acts of administration — meaning any one of the managers can, on his own, carry out the day-to-day acts of running the business. He does not need to consult the others first. This is the default the law supplies for a shared-management arrangement that the partners left otherwise unstructured.
Opposition: the majority decides
The right to act alone is not a right to override the others. The article continues that if any of them should oppose the acts of the others, the decision of the majority shall prevail. So a managing partner can veto a proposed act by opposing it before it is done, and once there is opposition the matter is no longer for one manager to settle alone — it goes to a vote among the managers, and the majority wins.
Breaking a tie: the controlling interest
The article also answers what happens when the managers split evenly. In case of a tie, the matter shall be decided by the partners owning the controlling interest. So if the managing partners cannot muster a majority either way, the deadlock is broken not by the managers as such but by whoever holds the controlling interest in the partnership — the weight of capital, in effect, decides. This is worth noticing, because it means ultimate control in a stalemate tracks ownership rather than mere headcount among the managers. A minority of managers backed by the controlling interest can carry a point that a bare number of managers could not.
If your firm runs this way
If your partnership has several managers and the agreement says nothing about dividing their roles or requiring joint action, this is the regime you are under, and it is worth understanding on both sides. As a manager, you can get on with ordinary business alone, but expect that a co-manager who objects can push the question to a majority. As a partner watching the managers, know that your capital weight matters when they deadlock. If you want something different — defined portfolios, or a requirement that certain acts be agreed by all — you have to write that into the agreement, because silence hands you this default.