Once the comments have been whitewashed then there is no way back to knowing who added what.

If you have two separate documents from the reviewers then you could reinstate the author names for each comment before combining. It is very fiddly so you may decide its not worth it.
I would do it by editing the comments.xml component directly:
- Make a copy of the file as a backup
- Change the file extension to .zip
- Open the file and go into the Word folder and find the file called comments.xml
- Open that file in a text editor and look through to find the value for w:author=.
- Do a search and replace for that value and replace it with your reviewers name.
- You might also want to set the w:initials value at the same time.
- Save and close the file and make sure the zip updates with that saved file.
- Rename the zip file back to docx

The relevant parts in the xml file look like
<w:comment w:id="1" w:author="Joe Blow" w:date="2018-06-05T22:41:00Z" w:initials="JB">

__________________
Andrew Lockton
Chrysalis Design, Melbourne Australia