You can't use two WAN NICs with the same gateway. It'll only route traffic out of the default gateway.
If you have more than one public IP address that can use the same gateway, you can do it with one NIC, and VirtualIPs and 1:1 NAT routing. (http://doc.pfsense.org/index.php/1:1_NAT)(http://doc.pfsense.org/index.php/What_are_Virtual_IP_Addresses%3F)
On my WAN2 interface I have a public IP address ...39, and ...40, and ...41. The ...39 is the LAN2 subnet. ...40 is a single IP on LAN2, and ...41 is another single IP on LAN2. I could easily have added another public IP and had that route to LAN1, but in my case I have a completely different WAN connection with a different (different than WAN2) gateway instead.
Create the one WAN interface and use 1:1 NAT with a VirtualIP.