diarchy vs undecimvirate

diarchy

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

  • Rule by two people. 

undecimvirate

noun
  • A group of eleven people, especially (politics) a council of eleven men sharing office or power and particularly (historical) the groups of eleven magistrates in ancient Athens. 

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