The problem is that both domains are being pointed at the same hosting folder location.
If you have 2 sites labeled siteA and siteB you should have a folder structor something like this:
/var/www/siteB these folders and the way domains point to them is called 'Virtual Hosts' You need to check your virtual host configuration and ensure that both directives are set and pointing in the correct location.
Depending on your Linux version the Apache configuration file may vary in location but typically its found in something like /etc/httpd/conf/httpd.conf or sites-enabled a Google search with your Linux version should give you an idea where your config file is located.
Once you located your virtual host file it should look something like:
There is many online guides on how to setup multiple vhosts using Webmin should you want some more advice other than my own a simple Google search should reveal a overwhelming amount.