be up to vs pitch in

be up to

verb
  • To do or be involved in doing; to do (often something mischievous or inappropriate). 

  • To depend on, to be the choice of. 

pitch in

verb
  • To help out; lend assistance; contribute; to do one's part to help. 

  • To begin briskly. 

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