diarchy vs polity

diarchy

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

  • Rule by two people. 

polity

noun
  • A politically organized unit; a state. 

  • An organizational structure of the government of a state, church, etc. 

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