configuring multiple sound cards in linux will depend somewhat on which version you're using. Here is some basic info for FC5 (as an example):
ccrma.stanford.edu/planetccrma/software/node5.html#SECTION00050800000000000000
My (meager) experience was that there might be issues with the order the cards are defined. My FC8 works fine as long as the USB card is the first (snd-card-0).