Gotcha.

It's interesting that there's no built-in way in Windows (that I know of) to bind a specific application to a specifc ethernet card.

However, I think you could accomplish it all in software, simply by using a piece of personal proxy software. Have the proxy port-forward the desired ports to the correct upstream router. Essentially doing the same thing as the hardware router, only doing it within your PC.

Hm. Maybe this could even be done without third-party software using Microsoft's RRAS features. Not sure. But I think you need to have a Server version of the operating system before those features are available.