It sounds like you're looking for a failover solution. A good place to start would be the following presentation by Andy Brist:
https://www.youtube.com/watch?v=KW5Qkl8brcA
After you've finished watching the video, you should have a good understanding of Nagios XI failover in general. My recommendations for active/passive failover are as follows.
Some key points to consider:
-Primary XI server must send a scheduled backup to the secondary server daily.
-Your second XI server will be restoring from the backup of the primary, which can be initiated manually or automatically (via event handlers or a script on a cron job).
-All of your agents must be accessible by both Nagios XI servers.
-All passive agents must be configured to send to both Nagios XI servers.
Deployment steps for this type of failover:
(1) Deploy and setup Primary XI Server.
(2) Configure Primary XI Server(Monitoring settings and so on).
(3) Deploy and setup Secondary XI Server.
(4) Configure the "Scheduled Backup Component" on Primary XI Server. Send the backups to the secondary server via SSH or FTP (SSH is recommended).
(5) Add a host check for Primary XI Server on Secondary XI Server.
Options for setting up the secondary server:
1. Do not run nagios on secondary and check the primary with a cron job. Start services only when the primary check fails.
2. Disable active and passive checks on the backup server and check the primary with a cron - when the primary server is down, enable all checks on the secondary server.
3. Disable notifications on secondary (allowing all checks to still run). When the primary is down, an event handler should be run turning on notifications.