Let $$\sigma$$ be the uniform surface charge density of two infinite thin plane sheets shown in figure. Then the electric fields in three different region $$E_I, E_{II}$$ and $$E_{III}$$

Field due to one infinite charged sheet is

and is directed away from the sheet.

Region I:

Both sheets produce field toward left.

$$E_I=\frac{\sigma}{2\epsilon_0}+\frac{\sigma}{2\epsilon_0}$$

$$E_I=\frac{\sigma}{\epsilon_0}$$

Region II:

Field due to left sheet is rightward and due to right sheet is leftward, equal in magnitude, so they cancel.

$$E_{II}=0$$

Region III:

Both fields are rightward.

$$E_{III}=\frac{\sigma}{2\epsilon_0}+\frac{\sigma}{2\epsilon_0}$$

$$E_{III}=\frac{\sigma}{\epsilon_0}$$

Thus,

$$E_I=E_{III}=\frac{\sigma}{\epsilon_0},\quad E_{II}=0$$