Manual:$wgSharedTables

Details
This only includes the user table by default for backwards compatibility. However you may remove it so that you can enable extensions using the shared DB without having shared users. Or you can add in another table, such as interwiki to force MediaWiki to use the shared version of the table instead.

Be careful with sharing tables. The images, revisions, pages, and many of the other tables may have bad side effects if you share them.