Mooneyboy
The answer is in auto flight chapter master FMGC logic. When both APs are engaged FMGC1 becomes the master and controls ATHR, Since FMGC1 ATHR channel is not available you loose ATHR and landing CAT degrades. It is good enough for CAT2 but for CAT3 you will have to use only AP2 which can use ATHR second channel.