Let's say I have one server with CPanel on it. I setup a second identical server and setup the 2 as a cluster for load balancing. If I am using RSYNC or something like it to keep the 2 servers identical, do I need a second full license of CPanel on the second server, even though it's only a mirror? If this does not make sense, please let me know.

Thanks.

Yes, you need a second license.