Sometimes we cannot think of the name of a person or the name of a place. We screw up our face, we ponder hard, and we try to recall it. Then someone suggests a name. We say "No, that's not it." How did we know it was wrong if we cannot say what's right? But then someone says the right name, and we immediately say "Hey, that's it!". If our brain knew all along what was right and what was wrong, why didn't it come into our consciousness when we were pondering? Do we have two brains, one which handles most thinking tasks but sometimes forgets and one which always knows the right answers but somehow cannot assert itself when we need it to? What's going on here?

We don't have two brains, we have much more! Philosophers (like Dan Dennett) and cognitive scientists (like Dan Sperber and Steven Pinker) argue that we have sub-personal processes that go on all the time in our mind/brain without the least control by our conscious processes, as when we parse a sentence in a natural language, a process that requires a highly sophisticated human mind but of which we do not have any conscious cue. The phenomenon you mentioned is at least similar to the "slip of the tongue" phenomenon, which is well known by psychologists (Freud also wrote an essay on it). There is a lot that is going on inside our brain and that we're not able to monitorate by our conscious processes.