Browse > Tags > Tag Sets > New Tag Set
In the Management section, fill in the same person for both Add/Remove Owners and Add/Remove Moderators, being mindful that is a thing and thus you might need to enter them in the format pseud (user)
Fill in a Title
Edit the tag set
Again in the Management section, fill in your co-owner/co-moderator's name to remove them
They should be removed as an owner and moderator.
They are only removed as an owner. You have to edit it again to remove them as a moderator.
This doesn't seem to be a problem if you're trying to remove different people, e.g. if you're removing testy (testy) as an owner and testy2 (testy2) as a moderator.