ChristiaanJ
Only in LAND mode could both APs be engaged at the same time, with normally no.1 flying and no.2 as a "hot" standby.
The system was referred to as "fail active", in that no.2 would already be synchronised to what no.1 was doing, and would take over totally automatically, without a hiccup (except an "oh merde" from the pilots, probably).
Your quote from memory is correct, although Land PRIME only was required for the second autopilot to be engaged.
Autopilot disengagememnts in Land mode werer in fact extremely rare. (Love the 'oh merde'bit though). The most comon autoland problems were the loss of Warning and Landing Display LAND 3 annunciation. Most problems were due to a failed flare test in the Pitch Computers at G/S capture and failures in the BCII inertial comparator.
Dude
