diarchy vs oligarchy

diarchy

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

  • Rule by two people. 

oligarchy

noun
  • A state ruled by such a government. 

  • Those who make up an oligarchic government. 

  • A government run by only a few, often the wealthy. 

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