Hello Rod, 

Failover functionality is included with an Enterprise license. You'll only need one Enterprise license, as the supported configuration of Failover allows only one mail server to be activated at a time. Your primary server will be activated with your Enterprise license key, and in the event of downtime, your standby server should then be activated. In the Failover documentation, there's a sample of a PowerShell script that can be created to automate the SmarterMail failover process (stopping primary and starting standby / stopping standby and starting primary). 

I hope this helps!