If you are using a WordPress Multisite installation and try to share items between sites (e. g. media or posts), we highly advise against this approach.
WordPress itself does not support “shared” items (posts, media, …) in a Multisite installation. By design, it is not recommended using a mechanism to share media between your sites. Each site in a multisite should be isolated and work without other dependencies. If you do so, you will run into issues when you start to migrate a site.
You are a developer? Currently, this feature is not on our roadmap, but we are open to contribution. You can checkout the complete coding of Real Media Library and checkout the developer api and make it compatible with e.g. https://github.com/humanmade/shared-media-library. Drop us a support ticket if you have any further questions.