Você poderia tentar algo assim ( /etc/network/interfaces
):
auto eth1
iface eth1 inet static
address 192.168.0.203
netmask 255.255.255.0
network 192.168.0.0
broadcast 192.168.0.255
up ip route add default via 192.168.0.201 dev eth1 metric 100
up ip route add default via 192.168.0.202 dev eth1 metric 200
Fonte: link