We use the Spanish tense pretérito imperfecto subjuntivo in certain dependent clauses and in particularly polite requests. We use the pretérito pluscuamperfecto subjuntivo to express actions that took place before a certain point in the past, or actions that could/would have taken place in the past under different conditions.

That's actually kind of a hard question to answer, even for a native speaker. The imperfect is just the simple past, like I went, had, was, etc., and the plusperfect is the "had had" tense referring to an event in the past preceding another event in the past you are talking about. So, it's "had gone," "had had" "had been, and so on. Casting both in the subjective case sounds odd to me. Sure. You can say 'would have had' or 'would have gone' or 'would have been' but distinguishing between the imperfect and the plusperfect is either impossible or extremely difficult (and very rare) in English. I can't even think of an intelligable example. Hence, I wouldn't even worry about it.