To create a qemu guest with more than one nic, just repeat the -net commands
qemu -net tap,vlan=0,ifname=tap0,script=./qemu-ifup -net nic,vlan0 \ -net tap,vlan=0,ifname=tap1,script=./qemu-ifup -net nic,vlan0 \ -net tap,vlan=0,ifname=tap2,script=./qemu-ifup -net nic,vlan0 \ -boot d -cdrom alpine*.iso}}
qemu-system-x86_64 rhel6u3.img -smp 2 -m 1024 -net nic -nettap,ifname=tap1,script=/etc/qemu-ifup,downscript=no -vnc :0 -daemonize