If you have a single slit it will produce a diffraction pattern where there will be maxima and minima in intensity.
So the light from a single slit has "preferred" directions of travel.
Another single slit close to the first one will also produces a diffraction pattern.
If the two single slits are relatively close together the two diffraction patterns from each of the slits will (almost) exactly overlap one another such that there are still regions where the light intensity is a minimum.
The important thing to realise is that presence of one slit does not affect the position of the diffraction pattern of the other slit.
You now have light coming from each of the two two coherent sources (the two slits) which overlap and this produces regions where there the light intensity is a maximum and regions where the light intensity is a minimum - the interference fringes.
In a region where there is no light coming from either of the slits there is no superposition of waves and so no interference.
The image below shows what happens in practice and you can see the intensity of the interference fringes modulated by the diffraction envelope and that the interference maximum at the position of minimum light intensity "missing" because no light is arriving in that region.