FWIW: We have several friends who use these on their tandems with one light each on the left ends of the bars. It makes for quite a light show; early-on we noticed that the stoker's bar end light looked like it wasn't on and made mention of it to the stoker who verified that it was. Turns out, her bar-ends were not parallel with the ground and the light was obscured the little hood that is intended to keep the light out of the rider's eyes.
Thus, the lesson learned was, if you use the bar-end lights make sure that the bottom bar-end of your drop bars are parallel with the ground, otherwise the LED may not be see as well or at all. Even having your bar-end pointed down a few degrees can abscure the visibility of the LED to motorists approach from behind.