How do I display a taxonomy term if another term is active for a node?
It was actually quite easy! No custom-coding required. I just added the token for the other vocabulary to the URL alias pattern, and it now appears, when it's present. When not present, the regular URL is formed.

So now the new pattern is [node:field_post_category:entity]/[field_focus_area:entity]/[node:title]

Drupal is great!