If you cross-post, note this fact both places. It may also be polite to cross-summarize in case points of significant interest get raised in one forum but not the other (but be patient on doing that, please). But even with that, I suspect you'll get some (minor) grumbling for cross-posting, even if it isn't done publicly.
You might get better luck serial-posting, if you have an unusual level of patience for a question-asker. Post the question one place, do your follow-ups there. After the thread appears to have thorougly died, if you still have doubts, then post "I asked blah over at blah and they said blah but that left me wondering blah and nobody there had an answer for that."
Demonstrating patience usually earns you less disrespect than not demonstrating it.