In editing mode, (if not before) a fiction writer (especially in romance) must answer/finalize two primary questions:
1.) Whose POV starts the story?
2.) When to introduce other protagonists or antagonists?
Hogan Kodiak in Sleepless in Montana has a problem, and he's perfect to demonstrate Who Controls the Story. Other family members revolve around him, and The Problem and sub-problem, but Hogan is central to the story.
A story will twist and turn as it is being written/emerges and characters/individual characteristics are born, edited, reborn, changed. To be creative means just that: you may not want to follow this rule and use another: What Works Works.
Sidebar: I just made a Quinoa and Kale Salad that did not work, also a doll's outfit that did not work, despite all efforts. So some things work and others don't.
However, here’s a good basic writer's rule of thumb (What is a "thumb’s basic rule"? Didn’t know they had rules :)):
Point 1: The starting POV should be that of Who Owns the Story. If one protagonist has a problem with another, the primary POV starts with that first protagonist. Generally, who owns the story, also finishes it. Always generally.
Point 2: When to introduce other protagonists/H&H/antagonists?
That can depend on the length of the story and the genre.
Today’s genre books can be much shorter than yesteryears’. They can have much more dialogue, which moves the story faster, and maybe a lot less color and use of senses. Media has influenced some readers’ need for give me quick, right at the start of the story. Building a story, making it live takes time. If space is short, the story has to move forward quickly to suit the readership.
I am reading a lot now, trying some new writers, and noticing less pinpointing, less color, less 5 senses. Yet the story is there, leading the reader on. However, as a reader, I need color (there’s that artistic personality thing again).
Putting the reader into the story, i.e. the scenery, the room, the location of said room is mega important, that old Who, What, Why, When deal. That can best be done in the lead POV and dialogue, jumping the story forward.
If the story is short, the second protagonist can be introduced after a chapter’s space break. If the book is longer, the second chapter or in yesteryear’s mega books, sometimes the third chapter. That is too long for my taste.
FWIW: If the second primary character is an antagonist, leading with the protagonist’s POV, an argument with the antagonist/dialogue, propells the story forward. Or introduce the primary’s problem, and follow with an antagonist’s POV, but the protagonist should dominate the story, good over evil and all that stuff.
Keep in mind that a story is fluid and characters sometimes make their entrance when they choose. Again, What Works Works.
But to me, as a reader, I need to be set into that story from the Get Go and that is in the protagonist’s POV or Who Owns the Story and What is the Problem.