# The loopback network interface
auto lo
iface lo inet loopback

# Network A
allow-hotplug eth0
iface eth0 inet static
address 10.1.10.100
netmask 255.255.255.0
gateway 10.1.10.1

# Network B
allow-hotplug eth1
iface eth1 inet static
address 192.168.100.1
netmask 255.255.0.0
gateway 127.0.0.1

ip route add 192.168.50.0/24 via 10.1.10.100 dev eth0
# This command makes things happy...

$ ip route show
default via 10.40.16.1 dev eth0
10.1.10.0/24 dev eth0  proto kernel  scope link  src 10.1.10.100
192.168.0.0/16 dev eth1  proto kernel  scope link  src 192.168.100.1
192.168.50.0/24 via 10.1.10.100 dev eth0

#/etc/network/interfaces

# Network A
allow-hotplug eth0
iface eth0 inet static
address 10.1.10.100
netmask 255.255.255.0
gateway 10.1.10.1
post-up route add -net 192.168.50.0 netmask 255.255.255.0 eth0
post-down route del -net 192.168.50.0 netmask 255.255.255.0 eth0

# Network B
allow-hotplug eth1
iface eth1 inet static
address 192.168.100.1
netmask 255.255.0.0
gateway 127.0.0.1

allow-hotplug eth0
iface eth0 inet static
   address 10.1.10.100
   netmask 255.255.255.0
   gateway 10.1.10.1
   up ip route add 192.168.50.0/24 via 10.1.10.100 dev eth0