Actually, the current E-ink tech is not two-sided particles--that was an older idea that was not manufacturable (the old "Gyricon" display). (It's still how artist draw E-ink paper, because it's easy to explain and east to draw). See the Wiki article:
http://en.wikipedia.org/wiki/Electronic_paper.
The E-ink used now has white charged particles in a droplet of black oil. The charge pulls the white particles to the top (for white) or bottom (for black). This means that when "white" thare is probably a bit of black oil left between the particles, thus the low brightness.
Unless you actually had a two-sided rotatable particle, it will be hard to get the brightness very high with this tech. Using a thinner oil might help. but I suspect this will cause fading to be worse--already an issue (see the posts on fading in sunlight, probably heat warming the display, making the oil thinner and allowing the white paticles to diffuse away from their position faster).
I hope the new iteration of E-Ink does more thaqn just use a thinner oil, for those reasons. What good is paper that fades away in less than a minute?