If a question is actually reworked such that an answer to one variant is not an answer to another variant (for whatever reason), then by definition it is not a duplicate, on-site or cross-site. (They can, however, very much be related, and I would encourage adding appropriate links.) Just make sure to attribute content appropriately. I can't see much reason why a question on Worldbuilding SE couldn't make people curious about how it would work in our real world and there is no reason to stop them from asking on the science-based sites such as Biology, Chemistry, Astronomy, Earth Science, Space Exploration, or what have you. I'm betting this happens already, but Worldbuilding SE is more likely to be a source of such questions.
However, cross-posting the same question to multiple sites is very much frowned upon on the network. At least one Stack Exchange site, the Unix & Linux SE, even has it as a specific close reason:
This question has been posted on multiple sites. Cross-posting is strongly discouraged; see the help center and community FAQ for more information.
The proper way to move a question from one site to another, absent established migration paths (which involve closing as off-topic, and which are added only if there is a significant number of questions which are good fits for specific other SE sites), is to flag the question for moderator attention and request migration to a desired target site. The moderator handling the flag is likely to seek guidance from the moderators on the desired target site as to whether the question would be welcome there, unless they are active on the target site as well and know whether it is likely to be, and then act on that judgement.
In fact, the latter (not cross-posting, and how to migrate) is currently specifically mentioned in the proposed help center "what questions can I ask here?" text. (In case it gets changed later, this is the most recent revision at the time which includes that text, and this is the revision that introduced it.)