Force traffic out through one WAN connection

We have two internal subnets, 192.168.1.x for protected internal and 192.168.168.x for public wifi traffic. The public wifi traffic uses separate physical connections (not VLAN) and ends up on a separate NIC on the untangle box. Right now, all outbound traffic, including public wifi, is going to the primary ISP. We have a lower speed WAN connection that is disabled.

I would like to enable that connection and force all (and only) public wifi traffic out through that WAN. How do I do that?