$${E_1 + E_2 = \frac{1}{4 \pi \varepsilon_0} \frac{2Q}{d^2} + \frac{1}{4 \pi \varepsilon_0} \frac{2Q}{d^2} \\ = \frac{1}{4 \pi \varepsilon_0} \frac{4Q}{d^2}}$$