1) Roman co-magistrate 
• (n.) One of two Roman officers or magistrates united in the same public functions.
Du·um'vir noun ; plural English Duumvirs , Latin Duumviri . [ Latin , from duo two + vir man.] (Rom. Antiq.) One of two Roman officers or magistrates united in the same public functions. 
duumvir, duumviracy, duumviral, duumvirate 1. In Roman history, one of the duumviri, the general name given to pairs of co-equal magistrates and functionaries in Rome and in her coloniae and municipia. 2. In modern use, one of two colleagues in authority.
one of two officers or magistrates jointly exercising the same public function.