If server 1 is not available then server 2 will be used for logins etc.
If the LDAP configuration is a failover configuration, then this is all you need. If both servers are available at the same time, then login queries may also be handled by the backup server, depending on the configuration and circumstances in the environment. As long as the LDAP servers are in sync, and using the same repository (or a replicated one), you should not encounter any difficulties.
The secondary (backup) server would only be referenced if the primary server was unavailable for 30 seconds, which could conceivably occur during busy periods or when network traffic is high, but otherwise only the primary would be referenced.
In both cases you should ensure that the host, port and base directory are specified, otherwise you can see a drop in performance when the secondary server is referenced.