Use Galaxy S2 / S3 as router for PC lan gaming
As near as I can tell, you need to run "ip route add" as root. I don't have root, so I can't try it. But something like this should work:

ip route add 192.168.42.0/24 dev rndis0 scope global proto kernel
ip route add 192.168.43.0/24 dev wlan0 scope global proto kernel


You can use ip route show to show the current routing rules.