The electric field due to a single uniformly charged infinite plane sheet is $E = \frac{\sigma}{2\epsilon_0}$ . For two parallel plates with surface charge densities $+\sigma$ and $-\sigma$:

1. In the region between the plates, the electric field due to the positive plate points away from it (towards the negative plate), and the field due to the negative plate points towards it (also away from the positive plate).
2. Since both fields point in the same direction, they add up: $E_{net} = E_+ + E_- = \frac{\sigma}{2\epsilon_0} + \frac{\sigma}{2\epsilon_0} = \frac{\sigma}{\epsilon_0}$ . (Note: In the outer regions, the fields are opposite and cancel out, resulting in zero electric field).