I think you are probably right. The max speed of the ADC that you have in the Mega is about 9.6 k samples per second - you cannot use the standard ‘discrete sample’ method with a diverter, so you would need to extend Robin’s or MartinR’s sketch to read all 11 channels and do the processing as well as control the diverter. At best, you would end up with a very slow (and inaccurate) sample rate for each channel - I think 14 samples per cycle.
Provided that you connect the two GNDs of the two Arduinos together, you should be able to share the voltage signal.
I presume this is the same house that you were asking about how the PV inverter shared its output?