I’d like to do that too—Zotero, which I used to use for PKM, allows you to do this, and it’s a great way to encourage serendipitous connections between notes.
My workaround has been to use the Folder Note plugin that @Archie mentioned, and then link each note to the (plugin-generated note of the) folder it’s contained in. If I want a note in two folders, I tag it with two folders. It doesn’t appear in each folder, but it appears in their backlinks, at least. This is basically a poor man’s MOC. Screenshot to demosntrate:
The note “Studiousness” is contained in the folder “Intellectual Virtue,” but it’s linked to both Intellectual Virtue and Academic work.
"Studiousness doesn’t appear in the “Academic Work” folder, but it does show up in the backlinks. Eventually I’ll update the Academic Work note itself to include all of those backlinks + descriptions, but this system lets me make connections between topics even when I don’t have time to update the folder note / MOC page.