bull session vs head-to-head

bull session

noun

  • An informal discursive group discussion, often one where politics, economics or current events are discussed. 

head-to-head

adv

  • Directly, one-to-one. 

  • Facing one another. 

adj

  • Direct, one-to-one. 

How often have the words bull session and head-to-head occurred in a corpus of books? (source: Google Ngram Viewer )