In my years as a volunteer book reviewer, I've come across a number of novels that attempt a Multi-Prespective Narration. This is when they have several protagonists and thus several viewpoints. If an author can make this work then it will enrich the narrative. If they fail, then it will be a mark against the story, bogging it down and/or crippling development. This article will illustrate the pros and cons of the approach with the books I've read serving as examples.
1. World building
When a story focuses on one character, that narrows the story's scope. Only what this character sees and experiences is captured by the author. This forces the story to focus on this character and their trouble. This can help the plot but what if the author is more interested in world building? By using a Multi-Perspective Narrative, an author can show more people in more areas with different lives, and thereby, develop more of their fictional world.
War of the Whispers book 1: Tears of Min Brok did a fantastic job on this point. It has three (later four) heroic groups traveling in three different areas with three different objectives. They can be seen as three "fronts" in this whispering war and thus a broader picture of the war is painted then what any one group could do alone. They will reference each other and occasionally interact, reinforcing this point.
There's also a group for the villains so the reader can see what they're up to when they're not fighting the heroes. This establishes them on their own terms instead of in contrast to the heroes. It fleshes out the war further. I gave this book a B.
2. Grey and Grey Morality or Right Hand vs Left Hand
When you have two viewpoint characters, then it's possible to have two "heroes" of the story instead of just one. If both of them believe they are noble and their actions righteous, then they can be foils of each other. In stories with a single viewpoint, this is harder to achieve because one of them has the sympathetic viewpoint and the other does not. It's easy for the former to come off as the truly righteous one for this reason. A dual viewpoint levels the playing field.
When I read Kindling Ashes, the Multi-Perspective Narrative that contributed to its A+ grade. You have an orphan girl firesoul who believes in the cause of the Firesoul organization, and a noble boy firesoul who wants to destroy the Firesoul organization and both of them have a sympathetic POV. The compare and contrast prevents one from declaring "Hero/Villainous" or "Heroine/Villain". (Except for the boy's older brother, who is an asshole).
1. Breaking Momentum
You have Character A off doing their thing and it builds up to something immersive and exciting. There's a chapter cliffhanger and the reader eagerly turns the page......to find Character B doing something; disappointment. Also, you as the author now have to build up action with this second character, and by the time you go back to the first, it feels flat even if the scene starts with excitement. When you have two characters, you have to juggle two series of events. This makes pacing the novel twice as complex.
When I read Dark Space 2, it had four plot threads taking place in three different places and two different time periods. It was like reading two books that shared a universe. One was simply superfluous, contributing nothing to the plot. It was confusing, boring, and made me impatient for him to make such division relevant. It was one reason for the book's low grade.
Conversely, Dark Space 1 used this technique with greater skill. There, the focus is 80-90% on the protagonist, and the rest is devoted to a couple of short scenes to deliver plot points crucial to the main plot. Without this technique, the author would have to cram these plot points into the main narrative somehow. It wouldn't be as neat or as effective.
2. Disjointed: Separate Stories Spliced
If you have two or more characters and they're not in the same group and/or location, then you basically have two stories in one book. You have to make them relevant to each other or the reader will think you spliced two separate stories together to increase the book's length.
When I read Tainted Dawn, I couldn't figure out why the three protagonists were sharing a book. They met once at the start and have little influence on each other from then on. One of them never meets one of the other two again and has little plot significance to the other. It was boring, tedious, and undermined the book's premise. This is the primary reason for the book's low score. Separated into their stories, they would have made fine stand-alones. In the end, I came to the conclusion that the author wanted to paint a picture of the era and the protagonists were mere character actors for this purpose.
3. Law of Conservation of Development vs Doorstopper
If you have two protagonists and two stories, then one of two things will happen. Either your book will be the length of one story and each of your protagonists will get the half the development, or your story will be as long as two in order to fully develop each protagonist. There are ways around this, but the fact of the matter is that you have to write two perspectives of the same series of events and that will make the book longer. Again, there's the possibility of becoming boring and tedious.
The Multi-Perspective Narrative is a literary tool. Like all tools it can be used and misused. The bottom line is that it has to be relevant.