If you are using a WordPress Multisite installation Real Media Library will have an own tree for each site. Media will not be shared between sites because it is generally against the WordPress Multisite approach.
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.