Question: Can a novel have two main protagonists?Answer:
Two protagonists within one story is not considered a viable option simply because they are redundant. Dramatica theory states that a character motivation/function should only be fulfilled by one character at a time. Pursuit of the story goal is the function of the protagonist
However, there are three ways to get around this aspect of the theory...
First, dramatica theory allows you to separate the roles of the protagonist (the person who pursues the story goal) and the main or principle point of view character (the character through whose eyes the reader views the story).
For instance, in some stories the reader may see the story from the viewpoint of the protagonist. In other stories, the reader may see the story from the point of view of the sidekick, the guardian, the antagonist, or any other character.
The second option is to have two main characters - two point-of-view characters - one of whom may also be the protagonist. This is trickier, because it is the main character's choice in resolving his/her inner conflict that generally determines the outcome of the story. Two main characters means two internal dilemmas to resolve. Often, only one of the two is the "main main character," whose choice determines the outcome. This leaves the second one's story not fully developed or less crucial.
Third option: it is also possible to have two stories within one novel, each with their own main characters/protagonists and story goals. The novel format is very flexible in this way. For instance, you might have several stories in a novel, connected only by the fact that they take place in one community, or within one family.