This is absolutely no different than Stack Overflow and the myriad of other CS-related sites.
There are some sites that have a lot of overlap with other sites, but the scopes of the sites are different.
On SciFi.SE, questions about Science Fiction & Fantasy in any media would be on-topic1.
On Movies & TV.SE, questions about movies and TV are on-topic1.
So the only area of overlap is Science Fiction Movies and TV. But how would you handle it?
- Do you allow overlapping questions to be asked on either site?
- Or do you decide that because SciFi.SE was first, than Science Fiction is their private domain and no one can ask a question on any other SE site that might be related?
You may also want to read Respect the community – your own, and others’ .
Ultimately, restricting the scope of one site because it is on-topic on another is not an effective way to grow a community, so there is nothing to do here. The question is on-topic on both and so you should not try to migrate them just because SciFi was "first".
This also means if you have a related question, you can ask it on which ever site you prefer. However, the choice of where to ask could also be helped by deciding what kind of answer you want.
- On Movies.SE, the community is built around Movie/TV enthusiasts, so you are going to get an answer from that perspective - people who love movies and/or television.
- On SciFi.SE, the community is built around enthusiasts of Science Fiction. Answers you get may go into more details into the lore behind a franchise, pulling in from other sources (such as books, comics, and related materials).
This premise is shared by other sites with some overlapping scopes. You are going to get a different community when you ask on another site. The expertise of that community is usually going to be different so they are going to provide a slightly different answer. It doesn't make it a better answer or worse, but just from another perspective.
1 - This assumes it meets the question quality guidelines