A government shared by two authorities is called a "diarchy". This term refers to a system of government where power is shared equally between two individuals or authorities, usually involving two heads of state or two branches of government. It is important to note that a diarchy is different from a monarchy, where power is vested in a single individual or family. In a diarchy, the power and authority of the government is divided and shared equally, providing a balance of power between the two authorities.