make pixels transparent when merging layers

I have two layers, Layer A and Layer B, both of them have transparent areas. I need to merge A into B so that the destination has the non-transparent pixels of A and transparent pixels where the same layer A has transparent pixels.

In other words, I want to draw my layer A into B but make pixels in B that correspond to transparent pixels of A also transparent. And I cannot select transparent areas by hand and then delete the colors inside the selection because the image is a drawing (like my avatar) and is too complex.