This is currently an involved manual process. Perhaps there is a way to automate it using a macro, but it fairly complicated, and needs access to both case metadata and comment metadata. This is similar to a Followup, but both cases are open at the time.

The flow is:

1. Select the comment. Note the User ID for the comment.
2. Add a new case.
3. Set the new case requester to the comment submitter.
4. Set the new case Subject to some function of the old case subject, similar to a followup.
5. Copy the comment from the old case to the new case.
6. Edit the comment in the new case as appropriate: copying any additional comments.
7. Copy the case metadata from the old case to the new case.
8. Add a comment to the old case saying a new case has been created based on the comment (comment snippet) from user (user name).
9. Save both cases

This can be quite a lengthy process. For example, a case may have 20 CCs, and it is much easier to delete them than to add them. I've tried forwarding the original email to clone a case in this way, and it's non-trivial to strip out the tracking headers that link an email thread to a particular case.

This enhancement would be a useful timesaver, and add a technique for forking flows rather than just joining them (Problem/Incident, and Merge).

If both cases could be edited before step 9, this would also be useful for workflows which produce cases for internal use: for sales, for internal discussions, and so on.