In WLC 4.2, there is only a field for the name of the Primary, Secondary and tertiary controller. If the AP only learns about one WLC during discovery, it can use these names for other WLC's, but as there is no IP address to be configured here, the WLC uses mobility group information to establish the IP.
Now, if the WLC's are in different mobilty groups. There is no way to provide this information via the GUI. A CLI command is required.
config ap primary-base WLC1 AP1 10.10.10.10
config ap secondary-base WLC2 AP1 10.10.10.11
An AP reboot will probably be required!