Get tags programmatically
Tags are embedded in script code and are tagged into the XML of a keyword test. So, you'd have to parse the text of both files to find tags, probably via some sort of Regular Expression, and extract them that way.
Personally... that seems a bit of a harder work than necessary. How about, instead of that, have the developers who add the tags, as a part of their development, insert a record into a data table that this other application that you're writing/working with can read to display available tags?