Joomla Multi Sites only share joomla the files and folder on the disk.
JMS does
NOT share the database content.
So the articles, news, forum, users, ... or whatever that is stored in the DB is
NOT shared.
See also FAQ
http://www.jms2win.com/faq#sec-73
http://www.jms2win.com/faq#sec-12
http://www.jms2win.com/faq#sec-72
Yes you can have
dummy1.net uses mainly uses tables with prefix dummy1_
dummy2.net uses mainly uses tables with prefix dummy2_
but they will
NOT share a table dummy_
Each website must be considered as totally independent.