Are you trying to set up some service that auto selects for nics on boot?
There are so many variables that I can think of on a nfs boot scenario, not sure I could even ponder a guess. You may have to describe each step on how you are booting this and to what.
I know that Knoppix had a deal where one can easily setup a pxe boot server and while in the setup of Knoppix terminal server, one can add in extra nic support. I'd look there for starters.