There is a long held mantra of screenwriting that there can only be one protagonist in each story. It is the character that undergoes the most change. By definition, the protagonist should occupy the most screen time.
More recently, such character structure is becoming skewed to allow for dual protagonists. That is, two characters, occupying similar amounts of screen time, each with their own character arc.
Ensure your story warrants dual protagonists rather than using them because you haven’t decided who your main character is.
Dual protagonists must equally spiral intersect and clash multiple times throughout the central story spine. Otherwise you run the risk of having two separate stories.
The concept of character duality is more closely linked to the opposing facets of the protagonists. This is related to the antagonist which ostensibly blocks the protagonist’s goal. They are often similar in that they both covet the same goal. Victory can only belong to one of them.
The dual protagonist can function as different aspects of the main character as separate dramatic units.
Dual protagonists often work in tandem; such as in romantic comedies and buddy movies. There is a shifting focus in the story as each protagonist pursues their own goal, complete with introductions, plot progressions, set backs and resolutions. Dual protagonists may be antagonistic to help each other reach their goals and evolve as characters, but antagonists are never dual protagonists.
Antagonists are always an external force and have less defined character arcs.
In a metaphorical sense, dual protagonists fulfil separate thematic functions within the same character. The duality represents the internal battle within your protagonist as represented by two characters. It represents the shifting dynamics of pain and suffering versus happiness and peace.
Duality also highlights the flaws in both protagonists. Each has something to learn from each other. This is different than the antagonist who is often static, but forces the protagonist to change.
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