This is controlled with backup settings on "Service Director > Virtuozzo Manager > Setup > Nodes Backup Settings" and "Service Director > Virtuozzo Manager > Setup > Default Backup Settings" screens.
Default backup settings will be applied for all newly registered Hardware Nodes. To adjust settings for old nodes, you may use 'Nodes Backup Settings' tab on hardware node properties screen. It should be enough to uncheck 'Allow to store backups at this Hardware Node' option for each node, and set 'Another hardware node' to a backup node.
Please make sure there is enough disk space in 'Directory to store backups' on new backup node.