The Independent Congress Doctrine

I have a simple-minded question. If there is a such thing as an "independent state legislature" doctrine, then does it not follow that there must also be an "independent Congress" doctrine? In other words, if a reference to "state legislatures" means that state constitutions and state courts cannot or are very limited in their ability to review actions by a state legislature because that body is specified, why would the same not be true for references to the "Congress" in the Federal Constitution? (Or, I suppose, textual references to the President.) Basically, this is a very robust departmentalism theory that must apply at all levels, no?