In cPanel, click on addon domains. You can use up to 2 addon domains. When you create an addon domain, it'll make a subdomain folder- you'll upload your other website there.
Only one account is permitted per person, however you can use an addon domain. This is configured in your cPanel and essentially points a new domain to a directory of your choice, effectively giving another public_html.
Please see the addon domain section of: http://x10hosting.com/wiki/How_to_Use_Your_Own_Domain for more detailed instructions.