I would vote for keeping them separate.
- retag-request is a very specific tag for requests of the form "Retag X as Y" or "Get rid of tag Z".
- retagging is much more broad, for all kinds of questions about retagging.
Look at these two questions:
- Find question for low-used tag and retag them.
- Is 200 reputation not too early for the right of retagging?