using a cylinder for a gaussian surface we find the flux through each end means that the surface integral will be double (the field has to be split between both directions, if you like)∮E⃗ ⋅d⃗ A=E(2A)=qϵ0=σAϵ0→E=σ2ϵ0which does not depend on distance from the slab.
The conducting slab in the middle picks up an induced charge from each charged slab, so it does not contribute to or hinder the field strength. Hence the total of the E-fields at any point should be∑E=E1+E2=σ12ϵ0+σ22ϵ0