I don't think IPNetRouter really does what you want at all... actually Apple licensed this software ultimately and incorporated it into OS X. If you look more carefully at the homepage, you'll see it has seen limited development since 2003.
In OS X, you can get an internet connection, say over ethernet, and then share it, say via Airport, with other computers, right? Turning your computer into a router. *That's* what multi-homing is for. The whole point of the IPNetRouter package was to function as a software router. Not what you're talking about.... sorry.... 🙁