you will have 2 servers with the same content, this without a active failover (this means that are manual steps involved).
For example web server 1 fails, if you use external dns / nameservers then you will have to modify the dns records off websites to the IP address of web server 2.
If you use the NS / dns in ISPConfig, then you will have to modify the settings in ispconfig on web server 2.
If you want an active failover you will need a load balancer before your webservers, the balancer will stop sending requests to one webserver of this one is down.
summary, ispconfig only copies data to another server and if you are using a x tier setup then the slaves only read out the database of the master.
ispconfig is not a failover solution