Where is the line between visualization and composition? To have the eye requires both skills, or something approaching them. To some extent the lack of composition skills can be substituted with formulaic devices and the lack of visualization skills can be compensated for by previewing tools like Polaroids. But where do the two skills overlap?

They overlap when you identify your internal perception of a subject and force that internal perception into the elements of composition. You may have a perception that a full range subject is really very bland and therefore decide to lower the contrast of the negative so as to emphasize the bland aspects of the subject. If you then frame the elements in the photograph fully cognizant that the final print will be forced to low contrast, you have worked on the join between composition and visualization. That suggests that you use "weight" of elements in the image they way they look to your mind rather the way that they look to your meter.

I think you have to really insist on the reproduction of the image you see in your head, as you see it in your head, before there is much overlap between visualization and composition ( or maybe that should be 'until there is much differentiation between visualization and composition'). I think composition implies that you arrange the elements that are there. I think that visualization, in the one sense, requires that the image be seen without concern for its measured light values. The values are measured after the image is visualized and composed with the visualized print values in mind; then the subject is metered and technical means are found to fit the metered values to the visualized values.