Doing something like this can be hazardous to say the least. You don't want the server switching over when you reboot, but you do want it switching over if, say the server is down for 30+ minutes.
You'll need to use something like rsync to sync data over from master to slave server. This will include cpanel data, homedirs, passwords, sql data, all of that.
You'll need dns entries on both servers, dns on the one should point to the one, dns on the second should point to the second.
You'll need entries in your nameserver configuration (at your registrar) to point to both servers. I'd setup 4 entries (ns1-ns4), the first two on the master server, second two on the slave server.
Make SURE you have the same MX entries on both servers, otherwise you'll receive mail on both servers, and if the first server is up, you'll always want mail going there, not to the secondary server.