congregate vs pair up

congregate

verb
  • To come together; to assemble; to meet. 

  • To collect into an assembly or assemblage; to bring into one place, or into a united body. 

adj
  • Collective; assembled; compact. 

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. 

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