Also there is a problem with your example sentences. It is not common to have 2 questions words used like that. It is possible that you could say: NANI GA DOKO DESU KA (What is where?)
But this would have to be midstream in a conversation as discussed in that grammar point linked above. I think with the sentence DOCCHI WA NAN DESU KA you were trying to say "Which one is what?" but this sentence doesn't work in Japanese. I am having a hard time coming up with a good example though of a replacement sentence, other than, KONO FUTATSU WA NAN DESU KA which means "What are these two things".