You should ask a question on WorldBuilding.SE if you are interested in the perspective of worldbuilders.
There are times when people can ask a question in similar form on multiple sites. But the different sites will always have a different focus. Physics.SE might very well answer you orbital-mechanics question if it's about real-world physics, but they might not like you describing fictional planets. On WorldBuilding you can also get frame-challenges more often that tell you "What you asked can be answered the following way: [answer] - but there might be a better approach to achieve what you want: [frame-challenge]".
There are also many topics that don't work well or at all on other sites. creature-design is a topic that comes up quite often here on WorldBuilding and wouldn't really be well received on Biology.SE or other sites on the network.
This site is also about stuff like the worldbuilding-process - which would definitely be off-topic on other sites.
All in all it depends on your questions. Maybe for your specific questions other sites would be more useful, but for many other people this site offers the kind of jack-of-all-trades thinking that they need to help them with their worldbuilding problem.
An important part is often how you phrase your question. Your example "What would a planet with two moons be like?" wouldn't work here because that's a lot of stuff you are asking about. So much that everyone could talk about about a different thing - society, physics, biology, ... - and all of those combined wouldn't be enough to sufficiently answer your question. But the question is: do you really need all of that information? What type of project do you have that you need to know absolutely everything about this setup and why can't you state some things and simply assume that's correct? For example most books gloss over... basically everything about the world, except for the things that are interesting to the story. Is your main character for example an astronomer? Then we should probably focus on astronomy.
If you have problems with phrasing your question you can check out the Sandbox on Meta, which is designed to help people that are having problems with their posts.
We are not a combination of SFF.SE and Writing.SE. SFF.SE is about canon worlds and only answers such questions, whereas WorldBuilding is about creating new worlds. Writing.SE is about help with the craft of writing, not with building your world or story.
In general we accept every question that aligns with our guidelines, such as:
- Must be specific and answerable: What problem are you trying to solve?
- Must include context: What are you trying to accomplish? Context gives people writing answers an idea of what your end state will look like and why you want to get there.
- Must include restrictions/requirements: What will make one answer better than another? If any answer is equally effective your question is not properly constrained. How can this be executed? What tech, timeline, magic or other criteria apply to the situation.
- Should include research: What ideas have you considered, or what information have you already looked at or failed to find?
A question should only be migrated if it is off-topic here and on-topic on the other site. You might get a few people that are saying you could also get an answer to a question on a different site, but, simply put, that's up to you:
What kind of perspective are you looking for? A scientists or a worldbuilders perspective?