I often talk in my head to work through a problem or something but I've never interpreted it as a conversation with a second presence. More like an inner monologue. When I was really young I used to think I could see myself as an observer. Now I think that was just a misinterpretation of your ability to picture your current facial expression. It's an interesting thought experiment but hard to say anything with proof, the brain constantly gives us feedback to our own actions. Probably as an extension of abstract social reasoning, the benefit we gain from being able to see how other people see us or may see our future actions, and on some level that feedback feels like real experience because it triggers some of the same neural structures that real experiences trigger. Like when you're playing a song in your head and it triggers some of the same brain activity as if you're listening to the song.