I have reproduced this issue in my lab - I assume you're attempting to use the following command to change the secondary Manager IP address:
set manager secondary ip X.X.X.X
This does not work because you have an MDR pair - it is the primary Managers responsibility to pass down the secondary IP to the Sensor.
I would recommend the following to potentially resolve your issue:
1. Reboot your Sensor and check the status with the 'show' command.
2. If the Sensor still does not inherit the correct IP address of the secondary Manager, try the following commands:
setup (enter all necessary information)
Please attempt the above procedures and report your results here. Thanks.
Technical Support Engineer
McAfee. Part of Intel Security.