IRule for sending traffic to another server if the first server is down or second server is down

Yes, it would be like a redirect traffic to this vip if server A and B are down.

Okay, so same priority group settings for servers A and B, and the following iRule:

when CLIENT_ACCEPTED {
    set default_pool [LB::server pool]
}
when HTTP_REQUEST {
    if { [active_members $default_pool] < 1 } {
        HTTP::redirect "http://foo.example.com"
    }
}

All traffic will go to server A until it fails, then to server B until it fails. If servers A and B fail, the client will be issued an HTTP redirect to the URL of choice.