I wanted to know what are the pros and cons of writing a book with two narrators?Answer:
The downside is that, whenever you introduce additional point-of-view characters (narrators) you weaken the reader's connection to the main character. One POV character only gives the reader the intimate experience of being that character. With multiple POV characters, the reader gets a much broader perspective on the story, learning things the main character doesn't.
The advantage of two POV characters depends on your genre. For instance...
In romance, it is common for a story to be told from the point of view of both romantic leads, so that the reader (typically female) not only puts herself in the shoes of the female lead, she also gets to know the male lead's mind, and know that he is the right person for the main character to form a relationship with.
In suspense stories, writers will often include the villain's point of view. Letting the reader see the villain gradually lay a trap for the unsuspecting main character creates the suspense.
In genres such as horror, dark fantasy, or thriller, it is also common to have the villain narrate, for similar reasons.
Also, sometimes a writer will use a secondary POV character in an attempt to appeal to a wider readership. For instance, if your main character is a brave, warrior type who will appeal to some readers, maybe you include a brainy, less confident secondary character who will appeal to a different crowd.