When I was active on deviantART, most of the stories I wrote had multiple viewpoint characters. There is a distinction between viewpoint characters and protagonists. Perhaps not a difference, though.
A lot of writing advice leans so far towards analysis that I don’t think any of it should be taken as is. That includes mine. I am incredibly analytical, and a lot of my conclusions come from completed works. (not all of them, though) I did a search about using multiple viewpoint characters the other day, and the results were depressingly unhelpful.
Small digression here: Some results were too specific. Related to the broad question, but questioning a specific scenario. One such brought up the question of having two protagonists on different sides of a conflict. The person answering this question ruled in favour of having one voice, as two diminished the impact. I disagreed, and immediately came up with a story that had certainly had two protagonists who were absolutely opposed.
Now, standing on the shoulders of giants really just sucks most of the time, so I don’t usually like to cite popular works. Especially when they break a lot of rules and got away with stuff for reasons. Death Note doesn’t really exemplify the scenario suggested. In fact, as some will have already loudly pointed out, there are not two protagonists in Death Note. There is a single, villainous protagonist. People just like to think of L as a protagonist.
That in mind, I thought up a better example, this one from whole cloth. Two princesses, sisters, marry at the same time. The men they marry rule warring kingdoms, and each princess tells the story of their own side of the conflict. Whether it ended in peace, death of one side, or mutually assured destruction, I think a competent writer could carry out such a premise to satisfaction.
What I was specifically looking for in my search was what people think of multiple protagonists as readers. I don’t mind them, but then, I rather enjoy writing them, and know that I do. I needed an unbiased opinion.
The analytical view prevailed. I don’t think I’ve ever personally known a reader to say they disliked multiple viewpoints, so it surprised me a little. But it seemed almost universal. Varying degrees and illustrations of, “don’t do it, guys.” I think there was the occasional caveat of, “unless the story calls for it,” but that is such a bum-cover.