What sort of traffic is it?
You are better off not policy routing (rules on gateway set to "default", not a specific gateway) in these cases. Though routing on WAN can be complicated sometimes due to how the outbound WAN rules have route-to on them. Maybe try an floating rule, outbound, quick, on WAN, matching the destination network and without a gateway set on the rule. You might also have to add a "do not NAT" type rule so that your private-to-private traffic does not get source NAT applied.
If the two internal networks are on separate VLANs within the same switch setup, also consider using a tagged VLAN as a means of handing off traffic between the firewall rules to avoid using WAN.