It mostly depends on the length of the first name. For example, I like
Ezra King because its short, each name has two syllables and it flows well.
Alexander might need a two syllable MN because it is long enough on its own - its just 3 syllables but it feels long to me for some reason. Maybe because it was a king's name? The last name also matters.
Alexander Keaton might sound good as is, but what
Alexander Jack Keaton,
Alexander Sebastian Keaton? It feels like a mouth full. I don't have a conscious rule about making combos. I usually don't consider a LN. Unless, the OP brought it up.
https://youtube.com/@MarioDrama632 or
https://youtube.com/@Objects_2763 :3