I'm working on something now that starts off with an obvious protagonist. Really, he's the one that the plot follows. Then he meets someone who seems to take just as active a role as he does in the main goal. Interestingly enough, but not at all surprising, it is a she. I did originally have this story in first person with him being the main character. Things were more clear cut then but now I have an entirely third person story with two characters sharing the protagonist position. Do I treat them equally or focus on the one that was first introduced? What would be the pros and cons of each?
A plot is defined by its conflict. Hoe each protagonist is tied in with the central plot (and its conflict) will provide the answer to your question.
Many writers switch between different perspectives in alternating chapters. EG. your first chapter may be wrote in the first person perspective using your male character, and your second could be wrote in the first person perspective of your female character. This method means that each of your protagonists have an equal share 'of the action' and readers see them both as equal in there goal, rather than feeling that the author is favouring one character in specific.
Whatever works best for the story and what you want to accomplish is what you should do. It's your story, so if you think it makes sense to focus on both equally, do so. If you think it makes sense to may more attention to one than the other, do that. Ask yourself this: Do you want us to know right away that the second character is important to the story? If you don't want the readers to know right away, I would say to pay more attention to the first one in the beginning. It will take the attention off character two so that if you want to surprise the readers, it's more likely that you will have that effect.
A good book I recently read with the same "issue" was Time Traveller's Wife. This book was written in first person narrative from both characters, simply by starting each narrative with the character's name and a colon. Another book I read a while back was House of Leaves where in essence 3 protagonists were layered into a single story, a first person writing and quoting another person's academically written book describing a third person's journey. Although one could argue that the three characters were equal in terms of development and that the protagonist is actually "something" else entirely - the narrative of the book certainly would work in your scenerio: You can stick to the first person narrative from 1 of the protagonist, but let him / her observe the other protaganist closely, writing about what they've observed. Their own story will still be told through the pages, maybe with more "psychological" development than "observed physical" development we see from the other character. You can of course still do third person and simply jump back and forth if you chose. The real question is - what will work with your story? The journeys that each of the characters take will more or less dictate whether it's possible or not to use each narrative. For instance duel-first person narrative will seem confusing if nothing connects the two together. In fact this brings up the point that your two characters' journeys will need to contrast or parallel each other to keep the flow of the story consistent, either drawing from the similarities of the nature their journeys, or the development of their characters, or just the events that they share. Finally, you need to also consider whether they're both protagonists, or whether one of them is actually the antagonist. Antagonist is not always a "villian", it's the character that embodies the "obstacle" that your protagonist has to overcome. And if they're neither "both protagonists" or "1 pro 1 antagonist", you'll definitely need to pick one as the protagonist and bring the other character in as a support character. Oh yeah, some films you might want to look at, off the top of my head: When Harry met Sally, Sleepless in Seattle, In the Mood for Love - a few romance films that balances two protagonists very well. Once Upon a Time in the West, The Good, the Bad and the Ugly, Requiem for a Dream, Silence of the Lambs - non-romance stories that draw very good balance between multiple characters, where all the characters are linked through events and their journeys. Hope that helps a little.
