Sure, it's possible to have more than one antagonist. Remember that the antagonist, by definition, is whoever or whatever is creating conflict for your protagonist. If your main character is "antagonistic" because of another person, then it sounds to me like that other person is the antagonist. It depends on your plot, though. If the conflict is due to something the protagonist did, because he dug his own grave out of his own stupidity, then the protagonist may be your antagonist.

Answer this question: When I read your completed novel, on whom will I blame the conflict? Will I be mad at your protagonist for getting himself into whatever mess he's managed to create? Or will I be angry at the other person, the one who caused the protagonist to be the way he is?

Wherever my anger/frustration lies as a reader, that's your antagonist. Your challenge when you complete your Contest Round piece, then, is to make me sympathize with that person, the one I would normally be mad at if I didn't have this back story to explain things. Make me understand his motivations, and soften my anger toward him just a little. Make me see where he's coming from.

Does that answer your question?