I'm going to go out on a limb here - but running 2 nics in one PC is a non-standard config.

Want proof? Go look at any desktop PC made by Dell, and find me one that comes with two onboard NICs standard.

I use multiple nics in one PC for two things:
1) FIrewall servers (Microsoft ISA, Soothwall, etc)
2) Redundant connections on server hardware.