A good question. I would love it if there were some way to have a question be on more than one site at a time, for edge cases like this that don't fit into a Stack Exchange bucket perfectly. Other examples:
- Writing a cookbook - could be on here and Seasoned Advice.
- Cooking while traveling - Seasoned Advice and travel.SE.
- Syncing a mac with an Android tablet - Ask Different and Android.
- This meta question could be asked here on meta.writers, Meta.SO, or meta.history.
...and so on. Trouble is, these questions would need to be at least somewhat on-topic on both sites.
How sites parse questions
The way each site reads, responds to, edits, or rejects a question is generally based on policies decided on by the community in meta. Each site has its own style, and different standards for how broad or targeted a question needs to be to get a good answer. So it'd be harder to find a way to make a single question work on more two sites than on one. Not impossible at all, but tough.
How could it be done?
If there were many questions that needed this kind of treatment, I have little doubt that a case could be made to handle it, and a proposal made clearly how to do it:
Perhaps a question could literally be on more than one site. You'd open a question about writing a historical novel, and there would be indicators that it was on both sites. Maybe there'd be two sets of tags visible.
If not, maybe a question could be asked on both sites, but edits would happen on only one site. The two questions would be linked - you'd still see it on both - but they'd diverge after a bit.
Both approaches could be valuable. Unfortunately, there would be issues to be worked out: Users with privileges on one site but not the other could be a problem. How would close votes be handled? Edits?
The issue here isn't so much that these things can't be worked out. I imagine they could. But it'd make Stack Exchange more complicated and more difficult for new users to learn, and the network already has a rep for being unfriendly and difficult to learn. It's probably not worth it for a few edge-case questions. If there are more questions like this than I think there are, someone can always bring this up on meta.SO and provide examples, making a case for implementing a system like this. But how would this help Stack Exchange fulfill its mission, to make the internet better? Unless there were lots of un-bucketable questions like this, and the proposal were to show a clear benefit to the network for doing this, I think this would be a lot of effort for very little benefit.
What to do now?
Right now, I'd recommend that you ask your question on both sites. Let both sites handle what they handle best. History will concentrate on the historical aspects, and Writers will help you with the writing. Both questions will probably be subtly different, and that's fine.