Adding multiple tags for a single highlight can be accomplished by using a comma (,) or the <enter> key. You can have several tags for any particular highlight you create.
If you edit a highlight in Study Notebook or by clicking the "Edit Note/Tag" link when you hover over a highlight in the scriptures, and then click in the Tags field, which has the icon

at the beginning of the field, you can begin typing your tag names. Be sure to use a comma to separate individual tags from one another. Click Save and you are finished!