I assume, that every of your net cards has different IP assigned. So, try "q3ded +set fs_game excessiveplus +set dedicated 2 +set net_ip IP.you.want.to_use". If it still does not work (because it can be like that - depends on your network settings) and the kernel spreads your bandwidth over the other cards, you will have to take a closer look at your routing rules and modify them a bit. If you want just to drop connections coming from other interfaces, block apropriate udp ports with iptables (look at 'man iptables'). If you want to play with routing, read http://www.policyrouting.org/iproute2-toc.html and (still excellent, although slightly outdated) http://lartc.org and 'man ip'. PM me if you have more specific questions (including info on your network settings).
AVE SATAN