"Throughout the period of time during which the plate of V2 is negative, the plate of V1 is positive". Can anyone please explain why this is the case?
The two plates (anodes) are driven by the opposite ends of a centre-tapped transformer secondary winding. The AC potentials at the ends of such a winding with respect to the centre tap are in anti-phase to each other. That is to say, when one goes positive, the other goes negative.