"It is possible to run multiple Plone sites within a single Zope instance while keeping each Plone site in its own Data.fs file. Having the sites in their own Data.fs file allows you to easily backup, delete, or relocate individual Plone sites even when you are running multiple sites in the same instance. This can be accomplished using ZODB Mount Points."

refer: http://plone.org/documentation/kb/multiple-plone-sites-per-zope-instance-using-separate-data-fs-files-for-each-one