Ollicrj: This is something that interests me because the original poster suggests this and you said this also.
I have tried this in 2 different simulators and each time with an a/thr channel 1 fail I can't get Cat3 dual back.
For a quick reference check, my company manual says:
"When both APs engaged in APP mode, no 1 ATHR is the master and controlling channel. If ATHR subsequently fails, ATHR channel 2 may be re-engaged after disengaging AP1. This will raise the landing capability from CAT2 to CAT3 SINGLE."
Because both my company manual suggests this and the fact I have seen it with my own eyes suggests to me one of two things. Either you are both mistaken, or there is a modification to the systems available that will allow ATHR channel 2 to operate with both APs engaged depending on engagement order. I will try to have a look through FCOMs tomorrow to see if there is an answer there.
Anyone else any thoughts?