I will phrase my question through an example. Consider this sentence in English:
I do not know whether you wrote where you are.
This has one governing clause ("I do not know") and two indirect questions, first subordinate to the governing clause, the second subordinate the first one. In Latin the predicate of a subordinate conjunctive clause follows consecutio temporum (sequence of tenses), and the correct tense depends on that of the governing clause.
The first question is governed by the present tense "I know" (scio) and describes an event before that of the governing clause, so I should use the perfect tense for "wrote" (scripseris). But which of the verbs scio and scripseris is considered to govern the second question? If scio, I should use the present tense for "are" (sis). If scripseris, then imperfect (esses). I am not sure, so I am left with two possible translations:
Nescio, an scripseris necne, ubi sis.
Nescio, an scripseris necne, ubi esses.
Are both of these grammatical and good style in classical Latin? If yes, is there a difference between the two?
In general, if a conjunctive clause is subordinate to another conjunctive clause, what determines the tense in the "lowest" clause? I would prefer to see examples from classical literature where such nested clauses appear.
Clauses subordinate to a nominal form were treated here, but the source used there does not discuss the present question.