You are not two different people but you have different roles throughout your day and life. Like there may be a childish part that loves to do fun stuff or a grown up part that makes sure you properly care of yourself and so on.

Your thoughts can be viewed as a kind of communication between these parts e.g. when you are conflicted because you'd love to watch one more episode of your favorite tv show even though it's late and you know you'll be tired tomorrow. On the decision making progress your different "parts" communicate and sooner or later you come to a decision.

Beside this situational communication there may be also things we internalized e.g. because our parents told us or because we came to the respective conclusion. Like "if I don't do my work properly, my coworkers will be mad".

Sometimes the relation between your "conscious parts" is off or the internalized thoughts are bad for you. That's why it can be good to be more aware of what you're actually thinking. It's just you in there, but a lot of you.