Configuration Assistance for WAN Failover

I understand how to do configure for basic failover. If WAN1 fails, use WAN2. I have a client in need of a little more complex config and not sure how to set this up.  Here is the situation: 

I want all traffic on WAN1 except for a set number of internal IP addresses that will go out over WAN2. If WAN1 fails, then the failover needs to move all traffic to WAN2. If WAN2 fails, then the failover needs to move all traffic to WAN1. 

Thanks in advance for your assistance!