This is discussed with Moshe, multiple names reflect multiple personality traits, mahbe coming from different observers. Not a given that they all come at birth, though.
In practice, even as parents get a navuah when naming kids, some might be better at this than others (like Manoach who could not figure out anything about the malach). So, giving the kid two names will give you two shots to produce something that corresponds to his personality and lifestyle. For example, a Bernard Feivel can be a politician with the first, or a yeshiva bochur with the second.