My understanding was that each AP must be able to conduct a full auto land individually. How else could each AP monitor and know that the other was doing the correct job. I think that the Master (FCC) actually does the job and the other slave just watches. If there is a disagree, a message is sent (somehwere) for the AP's to disengauge.
What I would be interested in is what you have to do to arm this function with just a single AP engauged(if at all)? Does it happen automatically?? I have never seen the (dreaded) uptrim come in when on a Single AP CAT I landing. Or as I supspect, the Auto land fuction is only armed when both AP's Engauge.
Of course I stand to be corrected!!
Mr L.