Greenhouse Recruiting doesn't allow you to directly merge two duplicate interview tags. However, by deleting one tag and reassigning current users to the second tag, you can simplify any repeated interviewer tags in your organization.
You can update interview tags on the Interview Tags page. (Configure icon > Custom Options > Interview Tags)
To reassign a tag, click Remove next to the tag that you want to delete.
Select the duplicate tag in the I want any entity using this interviewer tag to use this instead field.
When you're finished, click Delete Interviewer Tag.