There is very limited support for redundancy. Nice/Witness are not yet using 4.1 AES clients, so they do not yet have the ability to admin a backup server in the client.
AES 5.0 will introduce a more complete failover. Until then, a backup server is just a spare, sitting on the net, waiting for you to change the client admin manually to use the new IP of the spare.
4.1 client should be supported by Nice on the Perform 3 product later this year, and Witness 7.8 will support it (GA in August). With 4.1, backup AES can be setup, and if link to primary is lost, app re-establishes a link to the backup without admin. It does require that the backup have a license that matches the primary.
4.2 will launch in May with ENTERPRISE WebLM functionality, so license could be stored on third server (non AES for instance). Then, active AES gets license from the third server, if it fails, the backup AES gets license from the third server. Will reduce need to buy dup licenses.