Seriously, it would not, if you have split personality's, truthfully you are one person with different mindsets, chances are, you're not gonna know who the other person is! So, you would not be cheating because there is such a low chance, that someone with split personality's knows who the other person is and wants to date them. There has been cases where one split knows who the main split is, but it's usually the same gender and they know that they are one person and they still have thoughts like a human that know it would be weird to date yourself. So in reality, no.