Change facial recognition on multiple photos

You will have to do it one-by-one, unless you have a Time Machine backup of your Photos Library from the time before the two People albums got merged. If you have a very recent backup, where the faces are still correctly named, restore the library from the backup.

The problem is, that Photos is using the named faces to learn what a person looks like, and as long as their is a person with a mixture of both your faces in the People album, it will keep mixing the two faces.

Previously, right after the upgrade to macOS 15 Catalina, Photos and mixed several people in my iCloud Photos Library. I split them using this procedure: How to Correct Merged People Albums - Apple Community

Basically, I used these there steps:

• I added the photos from the two wrong People albums to a standard album, so I could find them easily again, after removing the wrong faces names. I also assigned a keyword with the name correct name to the photos of each merged person.
• Then I rejected all names in both People albums with "this is not". or "xxx is not in this photo". There must not remain any traces of the two people in photos, neither correctly nor wrongly named faces. Photos has to forget what it has learned about these two people to rest the face recognition for them. If you are using iCloud Photos the names have to be removed on all your devices and removed from iCloud as well.
• then I invented two slightly different names for these to people. We have to use a new name that is not associated with the contact card of these persons.
• Now I named the photos of both persons again, using the new name and checked continually, if Photos auto magically added a wrong face to the album. If yes, I corrected the wrong name instantly. You will occasionally see messages like "You confirmed three faces and seven have been added automatically". This is the moment to check for wrong names and to correct them instantly.

Once the two albums are correct again, make a new backup of your Photos Library, stop save your work and you can revert easily to this state, if the problem should occur again.

The additional keywords with the name of a person make it easy to check with smart albums, if Photos has lost a named face or merged two people. To see, if a face is missing g or wrong, I use a smart album with the rule like this:

• Match all "person > is not > Granny" and "Keyword > is > "Granny"

or

• Match all: "person > is > Granny" and "Keyword > is not >"Granny"

See; How to use Keywords to Back Up the Named Faces in Photos and Improve the Search for People … - Apple Community