Halftone Screen & Transparency

I have imported two grayscale TIFF photos (of silhouetted objects on a white background) into Freehand. I then applied the "Halftone Screen" round dot pattern each. My problem is: I'm trying to overlap the two images without the white backgrounds showing -- in essense, I want to apply a "Multiply"-like filter effect (used in Photoshop/InDesign/Illustrator). The "Transparency" option isn't working the way I want -- it screens back the halftone images. Rather, I'm trying to retain the values of the halftone screened image, while allowing the second image underneath to be visible (for example, as if you were placing blue acetate over yellow acetate to creating a green color where the two overlap).

In addition, is it possible to raster or convert a Halftone Screened image to vector art?

Many thanks!