SpeakNoWords
No, but that's not going to happen. It's just that the person would be eligible for both categories.
It's all crap anyway - a few years ago a dog nearly got nominated for Best Supporting Actor, and there's loads of actresses who have won/been nominated for Supporting Actress despite being the main/only female in the film.
It's all up to the studios anyway - they choose what category they want to lobby for. It's why Mark Rylance was in the supporting actor category despite being the co-lead. It was much tougher competition in the Leading Actor category, so they put him in the one with the best chances of winning.