Right now, if you publish on Spiral and cross-post to Cupid, you can’t select a story category from Cupid. As result, your story will be invisible on Cupid if anyone sorts by a native category there.
I suggest that when an author toggles ‘yes’ for any Addtional Publishing Site, it immediately displays the categories native to that site for the author to select from.
I think that it’s fine to put the other site’s categories as a tag actually but I can see the appeal of the story presenting different “sides” to the different sites. When I was cross-publishing my stories from GSS into GCS I actually changed the categories on a number of my stories by accident because I was doing it from the GCS side of things and it erased the GSS category tags that had been there originally. I don’t think it’s necessarily worth a higher programming burden on Martin if it’s complicated to implement, but it was something I assumed I’d need to do when crossposting in the first place until he told me otherwise.