Lets say I have a positively charged conducting sphere of Q. The electric field outside the sphere is

$$E = \frac Q{4\epsilon\pi r^2}$$

Now suppose this sphere is enclosed inside another hollow conducting sphere of -Q charge. By Gauss law, the electric field is still the same?

Won't the negatively charged outer sphere reinforce the electric field, causing it it be larger?

In another example,

The field from a infinite sheet of charge is

$$ E = \frac \sigma{2\epsilon} $$

But the field between 2 oppositely charged infinite sheets is

$$ E = \frac \sigma{\epsilon} $$

Why is this not the case above?