At this point, there are many Stack Exchanges, and overlap in topic is inevitable. For example, https://chemistry.stackexchange.com/ and https://physics.stackexchange.com/ both talk about quantum mechanics. (Similar examples exist in databases, programming, etc.)
Question migration works very well when a question is posted to a site, but another SE site is probably a better "fit."
But there are obviously many cases where more than one SE community has good fit. For example, this question on Physics.SE clearly fits both chemistry and physics communities. Moreover, it would probably benefit from contributions from multiple viewpoints.
So I'm proposing a new feature. Rather than simple migration, perhaps another option would be to link or share questions on more than one SE site. The question and answers would show up on both sites, and there would be some information box indicating that the question was linked.
Badges and reputation points would be given based on the site used to access the question (i.e., the URL). It's just a way to encourage multiple viewpoints on interesting questions and will likely drive more traffic to these questions.