Subsites with independent contents
You have a website http://master.com and want to have a subsite http://slave.com or http://slave.master.com which has own contents.
Subsite should be created as parked domain for master site in your hosting panel first.
Both master site and subsite will use own database tables and hence have different separated contents.
Note that files are shared, so if you de-install an extension on any site - it will be also missed on all sites because files are removed.
Joomla update, extensions install and update is performed separately on each site. There can be issues if extension doesn't 100% use native Joomla installer logic.
This is not the best configuration, you should clearly understand how different tables can affect same files.