How can I show two users what taxonomy terms they have selected in common?
You are looking for Taxonomy Filter module


  if vocabulary A is set to be 'filtered' by vocabulary B (which may be
  vocabulary A), then whenever someone views the taxonomy listing of a
  term in vocabulary A they will see a 'filter' menu of terms from
  vocabularies A and B. This allows the visitor to narrow down the
  listing by choosing another term - effectively showing them the
  intersection set of nodes from the two terms.