When using AuthAnvil for VSA you have the ability deploy agents using a Primary and Secondary SAS URL.
Using the AuthAnvil Addins for VSA
If you open VSA > Two Factor Auth > Deploy Agents - Select "We wish to configure a second AuthAnvil Server for redundancy". By Default the Secondary SAS URL is not displayed.
This will allow you to set the secondary SAS URL.
Using the AuthAnvil Management Pack
As the AuthAnvil MSI packages are not designed to use the standard silent mode installation options, you need to create a special INI file that the MSI will read during installation. This INI file must be copied to the VSA Server’s shared files directory before the script is scheduled.To build this INI file, you can use the LogonINIBuilder.exe, which is located in the “Management Tools” directory of the management pack.
Once you have configured the settings for the INI file, press the Create File button to create the ini file.
Note: If you do not have a secondary AuthAnvil server configured for redundancy, set the secondary SAS URL to be the same as the first server.
For an explanation about the difference between the AuthAnvil Management Pack and AuthAnvil Addins for VSA please see this article.