Once the Romans expelled their kings, they needed to decide who would now lead them. Tarquin the Proud had so greatly ruined the concept of kingship, though, that the Romans developed an extreme hatred of the very name of king, and refused to ever allow such a person to rule them again. In order to prevent any man from ever again acquiring a kingship, the Romans set up the consulship as their executive.
Two consuls shared power, each equal to the other, and if they disagreed, no action could be taken. Additionally, their appointment was only for a year, after which, they were not supposed to be able to take office again for a decade, a rule that was frequently broken later in the Republic.
Nevertheless, the consular system persisted for centuries, and was one of the most stable systems of government in the ancient world, a superlative it shares with the Spartan dual kingship. Perhaps having a double executive is superior to a single executive?