Under System → System Setup →
System Wide Settings, add both the main and backup machines'
IP addresses to Cluster Members (one per line). While not
strictly necessary immediately, this keeps the setting consistent
with the backup machine, in case the two switch roles.
On the same page, under System Replication Settings /
Targets, add a target, and enter the backup machine's IP address.
This will begin to send settings changes and newly walked pages to
the backup machine.