If my question applies to multiple SE sites (e.g. EL&U and Writers), is it ok for me to cross-post the same question to these sites, or is that practice frowned upon?
Cross-posting is strongly discouraged.
However, if you feel a question is relevant to two different communities, provided you edit and tailor the question to each community, it can be OK to ask in both places.
It is very important that you actually phrase and structure the question to target it specifically to the community of writers vs. the community of English language wonks.