I would do it without using the backup interface feature. Use floating statics:
ip route (corportate address) serial0
ip route (corportate address) serial1
ip route (ip address of corporate dialer interface) 255.255.255.255 dialer1
ip route (corportate address) (ip address of corporate dialer interface) 200
As long as either serial line is up, no traffic will be sent to the dialer interace.
If you are using a routing protocol, skip the first two static routes and make sure that the static to the corporate network via the dialer interface has a higher administrative distance than that of your routing protocol.
HTH
Mark