Personalized Books for Siblings and Twins: What Actually Works
The whole appeal of a personalized book is exclusivity. This one is yours. That is exactly what makes it awkward the moment there is a sibling watching you open it.
There is no single right answer here, but there are answers that work and answers that reliably backfire.
The thing that backfires: one book, two names
The instinct is to put both children in one book and call it fair. In practice one child is usually the protagonist and the other is a supporting character, and children detect this instantly and without mercy.
If you do put two children in one story, they need genuinely equal roles, and you should expect to be asked which one is the main one. Have an answer.
What works: one each, same day
Two books, opened at the same time, is by far the cleanest outcome. Each child gets the full "this is about me" moment and neither has to share it.
It is more expensive, and it is the version that actually produces the reaction you were imagining when you bought it. If budget forces a choice, this is the place to spend rather than upgrading one book to hardcover.
Twins specifically
Twins are the one case where two separate books matter most, and where parents most often try to economise with one shared book. Twins spend their entire childhood being treated as a unit. A book that is unambiguously about one of them, individually, is doing something no other gift in the house is doing.
Give them different stories, not the same story with a different name. That difference is the gift.
Blended families and step-siblings
The failure mode here is a book that quietly encodes a family structure someone is still adjusting to. Be deliberate about who appears and how.
The safest approach is individual books for each child, so nobody's place in the story is a statement about their place in the family. If you do want a shared story, talk to the other parents involved first. This is one of the few gifts where the framing can cause a problem, and it is easy to avoid.
An age gap of four years or more
If one child is nine and one is four, buy for the four-year-old and buy the nine-year-old something else entirely. The older one has usually aged out of picture books and would rather have the thing that matches where they actually are than a book they will politely thank you for.
The exception is a keepsake with a strong inscription. That works at any age, but it is a different gift and worth framing as one.