In this example, "The more tired you are, the harder you concentrate" should actually be "The more tired you are, the harder it is to concentrate" because the way it was written above means that when you are tired, you concentrate more, which to me does not make much sense since when I am tired I want to sleep haha.
Anyways, regarding your question..in this sentence pattern, you can use both "you" and "it is to"; it will just depend on the sentence. For example, I can say "The harder you study, the more you learn" but another sentence can say "The more you know, the harder it is." There isn't really a specific patter here..you just have to go with whichever sounds right ^^