decemvirate vs diarchy

decemvirate

noun
  • A group of ten people, especially (politics) a council of ten men sharing office or power and particularly (historical) the groups of ten men who reformed and codified Roman law circa 450 BCE. 

diarchy

noun
  • A state under the rule of two people; the form of government of such state. 

  • Rule by two people. 

How often have the words decemvirate and diarchy occurred in a corpus of books? (source: Google Ngram Viewer )