Van Der Graff question?

We know max voltage attainable by VDG is proportional to sphere radius and inversely proportional to breakdown of surrounding air, as nicely explained here https://www.youtube.com/watch?v=yaeITv9Ytko Question is does adding external capacitance to the dome increase max V. We can add capacitance by putting another sphere against the main one which is effectively doubling it's size and voltage, we can other sphere at distance connected by wire, another sphere can also be a plate of regular capacitor.. Do you see where i'm going. As far as i know adding such external capacitance, that is, charging capacitor(s) with VDG does NOT increase it's max voltage, but why not. According to logic it should as we are effectively increasing the dome size, doesn't matter if dome is all one piece or two pieces connected together.