Configuring a machine to switch packets between two Ethernets is pretty straightforward. Assume we're back at vlager, which is equipped with two Ethernet boards, each being connected to one of the two networks. All you have to do is configure both interfaces separately, giving them their respective IP address, and that's it.
It is quite useful to add information on the two interfaces to the hosts file in the way shown below, so we have handy names for them, too:
191.72.1.1 vlager vlager.vbrew.com
191.72.1.1 vlager-if1
191.72.2.1 vlager-if2
The sequence of commands to set up the two interfaces is then:
# ifconfig eth0 vlager-if1
# ifconfig eth1 vlager-if2
# route add brew-net
# route add wine-net