Bit of a wobbly question from the JAR people, I think not entirely correct ( having just asked our tech people ). Both FMC's cross-ref both ADC's, but take primary info from their respective ADC. Normally, the LEFT FMC is the "master" and the right is the "slave" for inputting data, etc. This is the other way round when the rt autopilot is in command or during manual flt with left FD off and RT FD on.
So, during ops with left or centre AP in command, both FMC's are primarily using LEFT ADC info. This was the best we could figure they were asking about.....
Clear as mud ,eh?