The WSA itself does not have the configuration for clustering or active/passive stand-by failover.
Depending on your deployment method, there are a number of methods to achive the similar objective from the configuration outside the WSA.
In the urgent need of getting the assistance, I suggest you log a case with TAC for quick clarification or contact your Cisco System Engineer, I guess as Cisco Partner you have access to these people.
I have already committed to Cisco presales and Cisco TAC. Cisco presales engineer said it should be supported but Cisco TAC said it doesn't support failover. Which one is correct?
Its all a matter of definitions.
What do YOU mean when you say "failover"???
What feature/behavior are you looking for?
Right now, when you have 2 or more WSA's in place, the two boxes have NO knowledge of one another... nothing is shared between them.... so they can't "failover" to one another...
Does that mean there isn't a "high availablity" solution? NO... but it means the boxes don't do it for you.
HA for WSA's is handled by the redirection method... eg WCCP, or PAC files, or a load balancer, etc. I think most people doing this use WCCP. The WSA's register with the router or firewall that they're there, and ready to take traffic, when one goes down, the router/fw sees that and just sends traffic to the remaining WSA...
I can't find the articles right now, but if you look through this forum for similar questions, you'll find responses I wrote with links to articles....