By analogy with a tag-based taxonomy (versus a hierarchical one), why not copy the question to the additional SE Site, leaving the original in place?
Clearly the boundaries that separate StackExchange Sites subject-matter wise are fluid (which seems like a good thing) and not clearly discernible--i.e., for cases near those boundaries the only way to know which Site has jurisdiction is to ask the question and see if it gets migrated.
Question migration is usually an unwelcome surprise to OP and to the answerers. Likewise, it's probably a headache for the network engineer--as evidenced by the fact that SE has been doing this for a while yet recent posts on SO Meta indicate that migration doesn't always occur cleanly, and it's not always done consistently across questions and users associated with questions (as OPs or answerers).
Putting extreme cases aside, when is migrating a question ever a good idea? Migration refers to taking something from one place and putting it in another. Unlike my library of Dr. Dobbs Journal, in which i have put each magazine in one stack or the other (by subject matter according to the article i'm most interested in) but one magazine can't be in both--SO questions are in the fancy new digital format, so they can be moved to another place and still remain in their original location.
Certainly this "copy over" scheme (i.e., migrate but leave the OP in place) most honestly reflects the reality of the overlapping and fluid boundaries between SO Sites.
But what about confounding issues like double-counting reputation, etc.? I can't imagine this would ever be a problem; every question has an one and only one originally-posted home Site.