Greenhouse Recruiting does not have an explicit merge feature for interviewer tags. Rather, we have built functionality into deleting an interviewer tag that allows your organization to reassign interviewers with the tag before its deprecation. This functionality essentially achieves the same goal of compiling interviewers from two tags into one tag.
Click the Configure icon 
in the upper right-hand corner and select Custom Options on the left-hand panel.
Click Interviewer Tags.
From the subsequent list of interviewer tags, identify the two interviewer tags that will be merged and select which of the two will be the primary interviewer tag (i.e. the tag that will remain) and the secondary tag (i.e. the tag whose users will be moved to the primary tag).
Click Remove inline with the secondary interviewer tag.
From the Remove Interviewer Tag dialog box, select the checkbox for I want any entity using this interviewer tag to use this instead: and select the primary interviewer tag from the dropdown menu.
Click Delete Interviewer Tag when finished.
The secondary interviewer tag will be deprecated and any users assigned the secondary interviewer tag will now be assigned the primary interviewer tag.