Its a VERY long time since I last flew the Trident, but my (hazy) recollection is that number 2 was actually not connected to the autothrottle. The reason was that there was not enough air coming out of an idling engine to keep the aircraft presurised, so on descent, we always had to keep number 2 at a medium thrust level (10,800 N1 or N2 maybe? - too long ago to remember). We never used autothrottle in the cruise, only in the final part of the approach. If the autothrottle was working hard, then number 2 had to be kept stable to avoid ear problems down the back. Rather unusualy, the NHP used to have his hands on the throttles for the approach (in BEA) and, for manual landings, would close number 2 when the HP called for power off. I can't remember what happened for autolands. I also flew the DC-10 later in my career and I was surprised and delighted to find that we could close all throttles at top of descent without loosing presurisation.
I don't remember any significant pitch coupling with centre engine power changes with either aircraft.