That's the point of Duolingo - if you were only learning useful phrases (where is the bathroom, my name is x, etc.) there's a chance you'd learn those phrases off by heart and not actually learn the rules of the language, whereas having to translate random sentences like this forces you to actually think about grammar and stuff! If you want to just learn phrases that you will definitely use, a phrasebook for tourists is much more helpful than Duolingo is. :)
It is two words. They have their own definitions and uses. Thank you is just a simplification of saying "I thank you." To thank is to give appreciation, and "you" is just a pronoun (In this case, an object pronoun). Missing a subject would be fine, but missing an object leaves a little in question, even with context. Missing BOTH, on the other hand, would make it especially weird.