How do I merge albums in Photos, especially two albums of the same person in "People".

If you are having hundreds of People albums and the distance for dragging is too long, you can also type the name of the person into the name field below the thumbnail of the album. Renaming to the same name will merge the two albums. I would pick a very distinctive key photo for the album you want to keep, so you can easily recognize this person in the list of suggested names, when you start typing the name.

You cannot merge standard albums, only People albums. To merge normal albums you have first to select all photos in one album, add them to the other album, then delete the original album.

Drag one onto the other