Figuring out who the main character is in your novel is often hard for the romance writer when both the hero and heroine are strong personalities. The same is true for fantasy novels with large casts.
The simplest way to find out is to ask yourself who has to change the most in very important ways to reach her/his goal. That person is the main character.
The main character should act to reach that goal, not have it happen to him/her as a matter of events.
Why do you need to know? If you know, you can make the novel stronger by emphasizing that character’s changes.
And when it comes time to market that novel to a publisher or the reader, you’ll know who to emphasize when you describe your novel.