Shutdown the stand by CPU (swinst, menu 1, 7, YES)
When it restart, login, and use the command "mastercopy -s NAME_OF_YOUR_MAIN_CPU"
Once the system restarted, go back in swinst and enabled the start auto start (swinst, 2, 6, 2, YES)
Try again your modification once the "twin" command says that everything is OK / READY