The kind of thing I'm describing is somewhat similar to combining an MAOI with a drug that's metabolized through a pathway that the MAOI suppresses: if you took either drug on its own, it wouldn't be a problem, but when you combine them, it's dangerous. I previously thought that a CNS depressant was a CNS depressant, but it sounds like that's not entirely true, since some CNS depressants inhibit respiration and some do not.