You could use wordpress multisite to do this. That setup is for advanced users tho, there is much that can go wrong, and you will need to troubleshoot on your own. Also since your sites are running under one wordpress engine and an "experiment" might knock your main site offline as well.
You can also create a second development site on a subdomain such as test.domain.com. You don't get any integration between the sites via the wordpress dashboard in this case, but you also won't have "an experiment" bring your main site down either. To do this add the sub-domain on the manage domains page in the panel. Then you can use the one-click installer to install a second copy of wordpress. When you set the sub-domain up in the panel be sure and use the SAME server "user" for the new test sub-domain (there is an advantage here when you need to copy themes/plugin files between the sites). I typically use .htaccess access control on the test site to keep nosy people, search engine crawlers, etc out of the test site.
Another difference is, if you set it up with multisite and ask support ANY question the answer you will get is they don't provide support for multi-site installations.