To find the overall intensity of the double-slit pattern, simply multiply the double-source intensity by the single-slit intensity:

I = I_max cos² (πd sin(θ)/λ) [sin (πa sin(θ)/ λ) / (πa sin(θ)/ λ) ]²

When we use the double-source equation to find locations of bright spots, we find that at some places where we expect to see bright spots we see no light. This is known as a missing order, and it happens because at that location there's a zero in the diffraction pattern.