In a Computer with Debian only one network card (eth0) is installed. The system will get a second ip address without installing a second network card. The second ip address will still be present after a reboot. It is also possible to assing a second ip address at the terminal so no reboot ist required.
Maybe you need this at a proxy server who usually has two ip addresses.
In /etc/network/interfaces add this in a way, that it matches with your requirements.
# This file describes the network interfaces available on your system
# and how to activate them. For more information, see interfaces(5).
# The loopback network interface
auto lo
iface lo inet loopback
# The primary network interface
allow-hotplug eth0
iface eth0 inet static
address 191.168.1.100
netmask 255.255.255.0
network 191.168.1.0
broadcast 191.168.1.255
gateway 192.168.1.254
# dns-* options are implemented by the resolvconf package, if installed
dns-nameservers 192.168.1.1
dns-search domainname.local
### additional entries
up ifconfig eth0:0 192.168.2.12 netmask 255.255.255.0 up
down ifconfig eth0:0 192.168.2.12 netmask 255.255.255.0 down
With these entries, the system is assigned a new IP at startup.
Manually assign the IP address on the fly; this gets lost when you restart the computer:
# ifconfig eth0:0 192.168.2.12 netmask 255.255.255.0 up
In a test it has even worked to assign a third IP address to a different subnet.