" OK, we know the 2 engines failed to respond simultaneously.
This is very strange, they are totally independent: only a couple of bits are shared, either fuel or throttle. "
The 2 FADECS for the engines are indeed independent but how about the autothrottle itself ? Is that perhaps a possible single point of failure ? Just asking.
Incidentally didn't early A320s have a software bug where the engines sometimes failed to respond to the throttles ? To fix it you had to close the throttles and then open them again.
Graham