My bad.
Must confess I didn't study regs regarding standalone CFII but I was made to believe from talking to other pilots etc in the past, that II is added after doing CFI. The practicality for flying as CFII but not CFI, is quite limited in helicopter world. Maybe that's why I haven't come across this exact info.
Teaching twin instrument would then be ME class rating and IR training, hence no ab-initio instruction, needing CFI. It does make sense. I guess I read too many different countries' regs. JAA/EASA would be an example why it'd NOT work. Though there's CRI/CRE.