Jet jocky
Your right there is no reason,on a 2 autopilot system if one A/P is u/s or an input to it. the Eng would downgrade the aircarft from CAT3 to CAT2.
There's a few permutations becuse Autoland is treated slighlty seperate to Autopilot by engineerig, you could have 2 sevicable auto pilots but both Rad Alts are stuffed, that would mean downgrade to Manual land. same for the ILS recvs and IRS.
As safetepee says you cant assume with all aircraft how system servicability is determined it can get quite complicated, thank god for the MEL!!!
Stu