How to separate two people mistakenly tagged as one in Photos app?

It will not suffice just to rename the incorrectly named faces. Once Photos's AI has learned that you and your son are the same person, it will remember it for a long time. This is sometimes triggered by photos showing the two people side-by-side, where photos could only detect one of the faces automatically. After a system upgrade, when photos is scanning the photos against may only detect the other face and use the only name assigned to the photo for the wrong face.

I am using this work-around, whenever this problem occurs, and this is best done on the Mac:

• First of all, I am creating a new album with all photos and videos of the merged two persons. That will make it easier to find all photos again, that will need fixing.
• Then I am creating a separate album with the photos for each person.
• Now I am removing the names from all photos of the two people using "This is not" and wait for the removed names to sync with iCloud Photos across all devices.
• Once the names have vanished from all photos. I am adding new, temporary names to the faces on the photos, some nicknames, that have never been used in Photos or my Contacts.
• When adding the new names, you can use the command "Confirm additional photos", but watch out for alerts like "You confirmed 5 photos, and 20 have been added automatically" or similar. When seeing such a message, check the added faces instantly, before any incorrectly named face will be used for learning what the person is looking like.

Good Luck, meldanes 😊

The "This is not" should suffice, if there are only very few incorrectly named faces, but one two people have become merged, we have to force Photos to forget this merged face, or it will be like a black hole, attracting everything.