It should be determined by the story itself i.e. who is/are the character(s) whose story is being told. But in reality it's more often about star power than anything else. Also the desi awards system have a hard time accepting more than one protagonist of the same gender.
For example, Hrithik received Best Actor nominations for ZNMD while Farhan and Abhay got Supporting Actor nominations) even though all 3 of them are the (male) protagonists of the story.
In Cocktail, Boman and Dimple are supporting actors. Saif, Deepika and Diana are the central characters. In fact Saif really is more of a supporting actor here while the two women are the main protagonists coz it's actually their story. But a "hero" has to be there (another problem with the desi awards/film system) LOL.
What about Karan Arjun? Did they nominate both Salman & Shahrukh?
Irrelevant question. Karan Arjun was not a nomination-worthy movie. 😆
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