Greenhouse Recruiting does not have an explicit merge feature for candidate tags. Rather, we have built functionality into deleting a candidate tag that allows your organization to reassign candidates with the tag before its deprecation. This functionality essentially achieves the same goal of compiling candidates 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 Candidate Tags.
From the subsequent list of candidate tags, identify the two candidate tags that will be merged and select which of the two will be the primary candidate tag (i.e. the tag that will remain) and the secondary tag (i.e. the tag whose candidates will be moved to the primary tag).
Click Remove inline with the secondary candidate tag.
From the Remove Candidate Tag dialog box, select the checkbox for I want any entity using this candidate tag to use this instead: and select the primary candidate tag from the dropdown menu.
Click Delete Candidate Tag when finished.
The secondary candidate tag will be deprecated and any candidates assigned the secondary candidate tag will now be assigned the primary candidate tag.