The monitor scripts executessp_companion...prepare_failback when you are failing back the TruCluster resource to the primary node.
To fail back to the primary node, make sure that the secondary companion is in secondary failover mode, and relocates the TruCluster resource on the primary node using:
caa_relocate server_name
Where server_name is the name of the primary companion, and host_name is the name of the host on which it is running.
To make sure that sp_companion 'prepare_failback' was executed successfully, search for this string in $SYBASE/$SYBASE_ASE/install/server_name_na.log:
SYBASE HA MONITOR: Prepare_failback was successful.
Before you relocate the TruCluster resource on the primary node, make sure that the secondary node is up and the secondary companion is running in secondary failover mode. If the secondary companion or secondary node is not up and running, do not relocate the TruCluster resource. If both nodes are down, restart the secondary node before restarting the primary node.