Just a quick note, if doing this on a router with a single internal interface.
Traffic originating on the local network will go out the default route of FIB 0.
Only traffic coming in on the second public interface will go out the same interface.
IOW, the setup above is really only useful for incoming traffic, to make sure that it goes back out the correct interface.
However, a few more IPFW rules can be added to classify traffic on the internal NIC.
It all depends on what you want to accomplish.
