Suppose the pair of plates is connected to positive terminal of the battery and the pair of plates Q is connected to the negative terminal of the battery.
From figure, it is clear that we have two capacitors C1 and C2. Positive plates of C1 are connected to positive plate of C2 and negative plate to negative. Therefore C1 and C2 are in parallel.
So, Cp=C1+C2=2C=2ε0ad