Hi,
You cannot configure a websense server as a active or backup.
e.g.,
url-server (dmz) vendor websense host 192.168.2.2 url-block block 50
url-server (dmz) vendor websense host 192.168.2.1 url-block block 50.
In the above configuration all the url-filtering will be taken by 192.168.2.2 if there is no response from 192.168.2.2 then the ASA/PIX will send the url-filtering to 192.168.2.1 websense server.
We can configure 16 url-server in a PIX/ASA.