I couldn't find any solution here to make it anyway (please direct me if there is existing solution somewhere).
Can someone please tell me if it is possible to load more than two masters in GECK at once (like O:A, and BS together)?

Ok. Sorry, I found it on wiki:

"To load multiple .esms in the GECK, you need to change this setting in the GECKCustom.ini file:

bAllowMultipleMasterLoads=1"