Cisco VPN Failover setup

I am trying to plan a VPN solution and need a little assistance. The
plan is to have Site A and Site B as main hubs running active
directory, exchange ,etc.

We have multiple remote sites with cisco 831s and the main sites with
3825s and one concentrator. The goal is to have a hardware VPN tunnel
setup with Site A and in the event of an outage to the connection to
Site A the connection to Site B would be utilized instead. Enabling no
down time for the end user.

What I don't want is traffic to Site B until Site A is offline. The
switchover must be transparent to the end user. I've tried to call
cisco but the rep was less than helpful.

Does anyone hear have any ideas or can lend a hand? Please let me