It doesn't always work. There are several places that may need editing depending on what you want to do. There are settings under devices-core for id'ing the NICs that are important when switching from 2 to 1 nic. There are settings in KDE Suystem that can be used to set and change ip addresses and there is the web panel and the file available from the startup app.
I have had the most success starting from the devices-core settings (be careful or you can hose the system) and then the admin panel and then restart the system. But to just set an ip for a nic that is properly identified the admin is the easiest and most straightforward.
I accidentally did a single core installation because I forgot to add the second NIC. And I already tried adding the second NIC lateron, configured it with the KDE system settings tool on the desktop but there was no eth0:0 there so I couldn't remove it. Then after a rebooting, LMCE was still using eth0:0 for the internal network, which is not what I wanted, obviously.
So now I found and changed "Device data/Network Interfaces", from: "eth0,dhcp|eth0:0,192.168.80.1,255.255.255.0" to: "eth0,dhcp|eth1,192.168.80.1,255.255.255.0", and that seems to have fixed it, thanks for the tip it saved me a reinstall.
After looking a bit closer I still noticed the eth0:0 interface, which I got rid of through commenting out anything eth0:0 related in /etc/network/interfaces (used the lanch manager), looks like this now:
iface lo inet loopback
iface eth0 inet dhcp
#iface eth0:0 inet static
# address 192.168.80.1
# netmask 255.255.255.0
iface eth1 inet static
With a bit of luck that will be the last of that.