Most of the discussion I have found is on cross-posting, rather than incidental duplication on different sites.
Overlap such as video game story/lore elements being on topic for both gaming.se and scifi.se is rather unusual, and generally the exception to the rule.
However, it seems there are some accepted guidelines that cover this situation. There are also a few grey areas, which I will outline.
Deliberately cross-posting is indeed something that is strongly discouraged. Part of the justification for this is that "99.99% of questions people have claimed as being cross site has been proven to be valid on a single site if written properly and thought through". I think that this specific scenario (scifi/fantasy video game lore) is that "0.01%". Both communities have explicitly stated that these types of questions are on topic.
Jeff's answer states pretty clearly that cross-posting is bad?
However, this meta discussion, created at around the same time as the previous one, has a slightly different take on cross-posting, claiming that it is okay so long as it is not "abusive".
Robert Harvey clarifies when cross-posting is considered "abusive". More importantly, he references a comment by Robert Cartaino:
Robert Cartaino is right; there's no concept of closing as a duplicate on another site. Down that road lies madness.
So... deliberately posting on both sites is probably not the best idea, unless you can tailor one question to focus on the gaming community's perspective, and another on the scifi/fantasy community's. However... two different users posting very similar questions on each site seems perfectly fine.