How to run torrent traffic on selected interface
as torrents software use broad range of ports, there is some dificulties to redirect all trafic to selected interface. but you can use adding second IP to your interface. then you must configure your torrents software use new IP (Transmission - with -a switch; uTorrent in Preferences/Advanced/net.bind_ip, etc.) and put rule on LAN:
TCP new_IP * * * selected_gw
then add NAT rule:
selected_gw TCP torrent_port new_IP torrent_port
and rule on selected_gw:
TCP * * new_IP torrent_port *
You can also make the torrent program to use a static source port for outgoing connections if the programs supports it (azureus vuze does for example), then you can match the traffic based on the source port and direct it to a specific wan.