Are you asking about Photos on a Mac or Photos on an iPhone or iPad? And which system version do you have installed?
As Photos has assigned the faces of both sons to the same person, it cannot name both faces in the photos of both of them together, depending on the system version. Starting with macOS 10.114 we could not name both faces, if a photo or collage has been showing us twice, for example together with our mirror image. Apple has removed this restriction in the most recent system versions, but in some older system versions it still applies.
it will be a lot of work to fix the merged faces album of your two sons.
It will not help to rename just the faces of son A and keep son B. The renamed faces will quickly be rmerged again with son B.
Try this:
- Collect all faces of your sons in a standard album, so you can quickly find back to them.
- Then remove all named faces from the merged People album with "This is not ...".
- When the merged People album is empty, name all faces of son A with some nickname, different from any name you previously have used.
- Now name son B as well with a new, different name.
- Make a backup when all faces have been named correctly.
- With for a few weeks before you link the named faces again to the correct contacts.