Configure WAN1/WAN2 Active/Passive?

[RichV]

Currently we have a Fortigate 50B that has a ipsec tunnel to our main office Fortigate 110C. The 50B has WAN1 with a 25Mb connection to the internet. We also have a T1 line back to our office that I would like to configure on WAN2. Then I would like to configure WAN1 and WAN2 in an Active Passive mode. I want everything to go over WAN1 unless the link goes down. Then I would like WAN2 to take over and pass the data over until WAN1 comes back on. What is the easy solution to use with the Foritgate 50B? Thanks in advance for any support/suggestions. Rich V