Setting up multiple physical servers

I’m trying to set up several physical servers using Webmin/Virtualmin and need some assistance with this. I currently have one server that’s been running for a year or so hosting multiple sites, tied to WHMCS on the front end to automatically provision the new virtual accounts, etc. I’m adding several new servers for specific hosting tasks. One new server will be for hosting nothing but Joomla CMS sites using CentOS 7 and PHP 5.5 and eventually PHP 7. The existing server is using CentOS 6.7 with php 5.4 and 5.5.12 currently and has several Joomla accounts that will be transferred to the new server. The rest are HTML only sites which will remain on it. I’m just confused about how to direct new virtual domains to the new second server (and any future servers as they are added). I’ve got one static IP that is visible from the interwebs pointing to my existing server. What, if anything, do I need to do or add to make this work?