Two-man rule

The two-man rule is a control mechanism designed to achieve a high level of security for especially critical material or operations. Under this rule all access and actions requires the presence of two authorized people at all times.

Per US Air Force Instruction (AFI) 91-104, "The Two-Person Concept" is designed to prevent accidental or malicious launch of nuclear weapons by a single individual.

In the case of Minuteman missile launch crews, once a launch order is received, both operators must agree that it is valid by comparing the authorization code in the order against a Sealed Authenticator (a special sealed envelope which holds the code). These Sealed Authenticators are stored in a safe which has two separate locks. Each operator has the key to only one lock, so neither can open the safe alone. Also, each operator has one of two launch keys; once the order is verified, they must insert the keys in slots on the control panel and turn them simultaneously. A total of four keys are thus required to initiate a launch. For additional protection, the missile crew in another launch control center must do the same for the missiles to be launched. As a further precaution, the slots for the two launch keys are positioned far enough apart to make it impossible for one operator to reach both of them at once.

On a submarine, both the commanding officer and executive officer must agree that the order to launch is valid and then mutually authorize the launch with their operations personnel. Instead of another party who would confirm a missile launch as in the case of land-based ICBMs, the set of keys is distributed among the key personnel on the submarine and the keys are kept in safes (each of these crew members has access only to his or her keys). Some keys are stored in special safes on board which are secured by combination locks. Nobody on board has the combination to open these safes; the unlock key comes as a part of the launch order (Emergency Action Message) from the higher authority.

...
Wikipedia