As Unbreakable == RHEL
then treat is like RHEL.
I've never used Unbreakable as I prefer RHEL/CentOS but with CentOS, you can install drbd & heartbeat to make a clustered pair.
I recommend that:-
1) You have a private 1GB Lan connection between the two Systems. This is where the Disk Device replication & syncing will happen
2) Have a second private LAN connection for the heartbeat messages
3) all external Network message use a third Lan connection.
4) I have found it easier to use a third system to configure the cluster (login 2 terminals via ssh) then you can see what is going in the logs and also with your scripts.
5) don't be afraid to wipe everything and start again if you get into a mess.
There are plenty of instructions for setting up drbd $ heartbeat on the internet so a little experimentation and if you are a reasonably competent SysAdmin then in a few hours you will have a working system.
Remember, Script Everything on both sides.