This is only 8 years late, but the calculations above are different.
Crosspower of AB / Autopower (AA)
FFT(B)/FFT(A) is effectively (BB)/(AA) or autopower (B)/autopower(A)
I believe if you look at how Fast Fourier Transforms, or in this case more likely Discrete Fourier Transforms (on a computer) are calculated, you will see that the amplitude is effectively squared, so AA or BB