Under System → System Setup →
System Wide Settings, add both the main and backup machines'
IP addresses to Cluster Members (one per line). Adding the
main machine IP will permit the backup machine to cooperate with the
main machine (e.g. when receiving settings/data from it). Adding
the backup IP will avoid needing to add it if the roles of the main
and backup machines ever switch (e.g. after a disaster and the main
machine is rebuilt and becomes the backup).
On the same page, set Disable All Walks to Y. This
will prevent redundant (and possibly conflicting) walks on the
backup, which are unnecessary because it will be receiving walk
data from the main machine.
On the same page, under System Replication Settings, set
Allow Receiving to Y. This will allow the backup
machine to receive replication data (i.e. settings and walk data)
from the main machine.