pair up vs unite

pair up

verb
  • To get into pairs; to join together to make a group of two. 

  • To put into pairs; to put into a group of two. 

unite

verb
  • To come together as one. 

  • To bring together as one. 

noun
  • A British gold coin worth 20 shillings, first produced during the reign of King James I, and bearing a legend indicating the king's intention of uniting the kingdoms of England and Scotland. 

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