I would argue that the exact same question can't be valid for both sites.
If this somehow is the case, the question is probably in a grey area and not explicitly in the domain of both sites at the same time. But in any case, such a question would be a rather rare occurrence.
More likely there may be facets of the question which should be separated and then posted individually to each site with links between them. Perhaps the preamble information is the same or very similar, but the ultimate question would be different.
I'm guessing the most likely candidate questions would be those about the worldbuilding-process. Those questions have the highest probability for a crossover with Writing, Role-Playing Games, or Super User.
However, our help section spells out some topics that could be belong under the tag worldbuilding-process, but actually belong on those other sites and not ours:
- General writing or storytelling (consider the Writers or Role-playing Games Stack Exchange sites)
- Software that doesn't directly relate to Worldbuilding (consider the Super User or Software Recommendations Stack Exchange sites)
Edit, given your comments:
For the specific example you may be referring to. While the question "How do I create myths?" would be valid here it wouldn't be on the Writing SE. Where "How do I portray the myths I've created?" would be appropriate there and not here. So, reposting the same question without changing the focus isn't appropriate.