They aren't distinct languages. In fact, they aren't even really dialects, they're just two ends of a spectrum. I guess the best analogue would be that in English, you can either ask for help, or you could demand assistance: they both mean the same thing, but the former phrase uses Anglo-Saxon vocabulary and the latter uses French/Latin vocabulary. It's the same with Hindi/Urdu: in many cases, you have two ways of saying the same thing, either using a Sanskrit word or a Persian/Arabic word. People who tend to have a preference for Sanskrit words are called Hindi speakers, and people who tend to have a preference for the Persian/Arabic words are called Urdu speakers, but a Hindi speaker will sometimes use a Persian/Arabic word and vice versa for an Urdu speaker.