I tried to add a second LAN addresses in CentOS 6.3 on a brand new install and it’s not working. I installed a new copy of CentOS 5.7 and tried the same and it worked right away.
Reason: NetworkManager is interfering with the settings.
Add the following to your interface configuration file for both the eth0 and eth0:0 devices to disable NetworkManager for this NIC.
NM_CONTROLLED=no