Why not just use rollover DNS resolution and mirror the accounts?
I would presume you already have the mirroring portion setup if you
already have the same content setup on both servers. In that case,
all you would need to do is have (2) TWO entries in the DNS 'A' records
for the same web address and you'll already have most of what you ask.
In example ...
Code:
www IN A 127.0.0.1
www IN A 127.0.0.2
(Surprisingly most people don't know that you can actually do that)
IE: instead of "www" on one server and "www1" and the other,
you instead make both servers "www" and update the DNS zone
files to have both IP addresses with (2) separate "A" records with
the same "www" host identifier.
Upon DNS resolution, BIND will normally rollover between the two
IP addresses at each query and basically but not exactly load balance
all you incoming connections between the two servers.
If one server is down, the DNS resolution will fail but having both entries
delivered will trigger most hosts to try the alternate IP for connectivity.
The one thing that would be important though is that the DNS is not
handled solely by the downed server. I would instead cluster your DNS
between the servers and set both servers as authoritative servers for
the domain name in question.
You can also update the ZONE templates in Cpanel if you want to have
the secondary IP address automatically added at account creation time.
If you need to access a specific server, you can either create an alias
address such as "www1" or "www2" or just add the IP of the server you
need to connect to the "hosts" file on your own local computer.
Hope that helps