For full redundancy, you need two Cisco LocalDirectors. The two LocalDirectors can be configured to back each other up, so that when one unit fails, the other takes over for it. However, they are only one component of a fault tolerant Web application system. See the White Paper: Failover Configuration for LocalDirector for details on how to set up a primary and a secondary LocalDirector.
When you have both a primary and a secondary LocalDirector, there is still only one virtual server and one virtual IP address. If one LocalDirector goes down, the other one assumes all of its configurations.