I believe this was explained several times, but basically goes like this:
ROMs 01+02 in CPS2 B Board --> ROM 01 in Multi (Sound Program)
ROMs 03 up to 10 in B Board concatenated --> ROM 02 in Multi (6800 Program Code)
ROMs 11+12 in CPS2 B Board --> ROM 03 in Multi (Sound Samples)
ROMs 13+14 in CPS2 B Board(padded with 0xFF if necessary to make each 0x200000 in size) --> ROM 05 in Multi (GFX)
ROMs 15+16 in CPS2 B Board(padded with 0xFF if necessary to make each 0x200000 in size) --> ROM 07 in Multi (GFX)
ROMs 17+18 in CPS2 B Board(padded with 0xFF if necessary to make each 0x200000 in size) --> ROM 09 in Multi (GFX)
ROMs 19+20 in CPS2 B Board(padded with 0xFF if necessary to make each 0x200000 in size) --> ROM 11 in Multi (GFX)