1. If your database can support the traffic ,you can copy options(this table contains some info of the site, such as web title ,themes,and so on) table for different site into same database, and use my plugin to read different operations when user visit. Through this we can make our site use different operation infos but same posts and users(example sites:
http://ostack.cn,
http://jike.in)
2.If your database can not support much traffic ,i think you should copy the database to another , in this way the site can not share the posts