How to share media between independent blogs?

I’m afraid you’ll also need to create the database records – that’s how WP looks for media files, starting in the db. Those records then tell WP the local path to the media file.

But that means that if your web server has access to both sub domain folders on disk, you don’t need to hard link anything – just create your media records in the second db using the first db’s path values.

All that being said, I should point out sharing resources between subdomain is probably not wise: at some point in the future, it is very much possible one subdomain will move to another server… Then your shared paths won’t work anymore. (My $.02)

Hope this helps!