Actually the Doctor Who analogy might not be so far out. If you were to take the sharper image and remove the color (converting it to grey scale), you could then use somethine like avisynth's mergechroma, mergeluma.

I've even used them with different aspect ratio sources before, you just tag on a resize to each clip to match them (with a bit of cropping as needed). It takes a bit of jiggery pokery to get them right, but it does work.

Like I said I don't know if you have anything like that for the mac.