If I post a UX-related bug or feature request on the main SO meta site, under what conditions should I also post it on this meta site?
Good question. The audience on Meta StackOverflow is far larger than over here so if it's a global issue occurring for all sites then generally I'd post it to MSO and maybe link to it in our [chat] site.
It's better to have a canonical question so that answers don't get spread across a range of questions but exist all in one place.
However, that's not to say you can't ask the same question in both places, but you should target it specifically to the audience of that site. So UX issues would be written with the focus on the UX impact if posted here and then focused on the general Stack Exchange community if posted to MSO.