It's because of the pad modules used by FMCB.
I think that FMCB v1.8 uses rom0:PADMAN, which does not support multitaps (At least, I don't remember that it does).
Adding support for multitaps will require some hacking of FMCB because it's a closed-source project.
EDIT: WOW! MY FIRST SUCCESSFUL POST SINCE LAST YEAR!
Finally, people can hear my "voice".
SCPH-10000 S. MINOKAMO v1.01 (Defunct)
SCPH-10000 S. KISARAZU v1.00 (faulty)
SCPH-15000 S. KOHDA (With warranty seal!) :D
DESR-5100 S. EMCS