I am not familiar with ISPConfig Replication feature. Can I use it to have 1 website hosted on 2 servers and have them load balance the website?
In general, yes. But it depends on how you set things up and on how your website is developed. What ISPConfig mirroring mode does is that it replicates the same configuration on multiple servers. So when you create a new website on server A, the same site with identical Apache/Nginx/PHP config gets created on server B. To use this for a load balanced website, you must take care that the harddisk partition which holds the website files must contain the same files, so you must use either a network filesystem for /var/www or another way to keep this in sync like using unison. Then your website likely uses a database, which means that you must configure the CMS system of that site to connect to the same database or database cluster. Now you have to systems that host the same site, so the final step is to put a load balancer in front of the servers to distribute the load to the two nodes, depending on how the CMS in that site works, it is better to configure the load balancer to stick each user session to the same server.
