It is not possible to un-merge any candidate profiles once they've been merged—whether automatically through Auto-merge or manually from a Duplicate tag.
All of the information from the secondary profile would be deprecated. However the resume / cover letter should still be found on the merged profile under the Documents section of the candidate profile.
If two profiles are merged that are not true duplicate candidates, the best resolution is to retrieve the resume / cover letter and re-create the candidate profile manually.