Hi Dan,
For testing, you can use the ip address add ... command to apply the second IP address to the interface like this:
sudo ip address add 172.16.0.20 dev eth0
The simplest way to make that persistent may be to add an @reboot cronjob running the above command (as a root cronjob, without the sudo).
It can be done using /etc/network/interfaces but there are some suggestions that this may be dangerous on some hardware/driver combos, but look at the iproute2 method found near the bottom of this page:
73,
Stefan MI0PYN
