The bottom line is this: Just as we live life through one viewpoint - our own - characters must live life through one viewpoint; their own.
Readers expect to be shown your story through the eyes and ears and mind of one character at a time because they want to experience the story just as they experience real life.
Just as you can’t be inside your own head and inside your best friend’s head in real life, readers will not put up with being forced into the brain of one character and then whisked away into the brain of character in the same scene.
It’s what the viewpoint character feels, thinks and believes that really matters.
In a novel, you can write scenes in more than one viewpoint, but one must clearly dominate the book. That’s because every story is ultimately one person’s story above all others, just as your life is yours and yours alone.
In a novel, the majority of the book should be clearly and rigidly told from the viewpoint of your protagonist. That character’s thoughts, feelings, perceptions, and intentions should dominate the action.
When you change viewpoints, if you must, it should be only at the end of a scene or chapter and only when the change serves the story you're telling.
My favorite viewpoint is called “third person, limited multiple viewpoints” (Don't memorize it; the concept is more important than the phrase.)
Using LIMITED multiple viewpoint, I can follow the killer, or the person who ﬁnds a body, or someone else important to the plot, when I want to, while still spending the majority of the time in the viewpoint of my protagonist.
In my ﬁrst three books, most of the chapters were told from Veronica’s viewpoint, in third-person, past tense, limited omniscience. But I went to other characters’ viewpoints when it served my purposes. I used their viewpoints to increase suspense and danger in the books.
With third person multiple viewpoints, I could let readers know there’s a man hiding behind the door when – of course - Veronica didn't know it.
That’s a great way to create tension and suspense.