Find the ini setting for loading multiple esms. Set it to 1.
And where would that be ?? -- the ini does not have any setting for esm (searched using find text and no such line - allow multiple editors only allows multiple copies of the CK to be open but I can not find any setting to allow the use of multiple esms it errors out saying "Multiple master files selected for load. Load operation aborted."
Have you actually been able to load more than 1 esm or are you just guessing ?
EDIT : Found out how -- you need to add bAllowMultipleMasterLoads=1 to the [General] section (the line is not there but you can add it and then the CK will allow multiple master files !