I saw a guy that had a white light mounted on the back of his rack, aimed at his back. From 50 yards back or so it was easy to see that it was a cyclist. But this required a fairly bright white light.

I still think the one high and one low offers visibility for a longer distance, but like I have said before... light up like an alien mothership... blinkies high and low and white light aimed at your back and lots of reflectors, white blinkies forward as well as a steady headlight... make yourself noticeable.

But at a minimum, one high and one low.