Use 2 values ​​of a mask

Suppose we have a black / gray / white mask and we want to draw the image A on the white / gray values ​​and the image B on the black / gray values ​​(the more the image is black, the more she is black). The best I find would be something like:

  1. Set on RenderTargetA
  2. Draw the mask
  3. Draw PictureA on a mask (with DepthStencilState)
  4. Set on RenderTargetB
  5. Draw the mask
  6. Draw PictureB on the mask
  7. Trace both RenderTargets one on the other

But that just seems a) too complicated and b) both make the targets always have the mask drawn on them so that it would not have worked anyway. So how would you get there?