while you can’t share multiple libraries, you can share groups of references, which is a part of your second request so you can share parts of your library online with others.
see http://community.thomsonreuters.com/t5/EndNote-General/Questions-about-sharing/m-p/126999#M23788 and
Here is the share description: http://endnote.com/product-details/library-sharing
Here is a you-tube video about sharing groups. https://www.youtube.com/watch?v=hSExPuxdbcM