Sogeking is not a split personality, he is a fictional character made up by Usopp, that conversation was between Usopp and him self, he wanted to know what Sogeking would do in his situation so he conjured up a Sogekin personality to help him self, this is purely mental and not a real condition. It's like when someone does something and argues with them selves in order to find flaws and discovere possible improvements. In Usopp's case he has set a role model for him self in his own mind and is able to act in character of that role model. Not a split personality.