I can give you a concrete example of where this is not true because the author went to a lot of effort to give both books open questions which are answered by the other: Vernor Vinge’s “A Fire Upon the Deep” and “A Deepness in the Sky”.
He made sure that the second book answers the open question(s) left by the first, but also that it had open question(s) which are answered by the first book. It really doesn’t matter which one you read first, because the other one closes the loop, as it were.
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