More than one person is required in order to enact a critical
function. Examples include controls to require that two people
simultaneously urn keys in order to launch a weapon, cryptographic key
distribution systems in which keys are shared among many individuals, a
subgroup of which is required to participate in order to grant access, and
voting systems in which a majority must agree before an action is taken.
Complexity: Although some such systems are quite complex, there is no
fundamental complexity with such a system that prohibits its use when the
risk warrants it.