C:/whatever/whatever/im_notNnear_myComputer_right _now/my documents/my games/GECKCustom.ini (your folder path tree is different)
(save backup for that file in case the cat jumps on your keyboard and you accidentally hit SAVE)
bAllowMultipleMasterLoads=1
I thought by now that should have been auto corrected
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From the guide
http://geck.gamesas.com/index.php/Category:Data_Files
To load multiple .esms in the GECK, you need to change this setting in the GECKCustom.ini file:
bAllowMultipleMasterLoads=1
This file is NOT found in the fallout3 folder in the BethesdaSoftworks folder. It is found in the user folders under
My Documents/My Games/Fallout 3
Further this file typically can not be found under a normal search in windows.
NOTE: If an enabled .esp has a master file, and that .esm is not enabled in the load order, the game will crash on startup.