Robin2:
I find your use of language a bit confusing. Your second paragraph conjures up one-hand clapping. But I think you just mean the more usual mode in which a single analogue pin is used to measure a voltage by reference to Arduino GND....R
The second one is what the OP's earlier schematic showed which is why I tried to explain why to connect the grounds, so the volts will be on the same scale, all relative to the same ground.