I wonder if it would really get implemented anytime soon. At first, I was thinking that two screens wouldn't be cost-efficient to make. Also that it might suck up more juice. But the idea of having both screens being touch-sensitive made me think of a possibility of a PSVita-style (touching the back while viewing the front) control for some apps. That is if one screen's touch sensors are active while viewing the other screen. Also, what developers would develop for a single or few tablets, unless it becomes as popular as the iPad, which I doubt.
Using the e-ink reader (reading books and stuff can take a while) a lot to save power compared to an LCD screen does sound reasonable though.
The ideas about having two screens to worry about mentioned by some commenters above are interesting, like having to clean two screens and having one screen placed on a surface all the time. They would probably need to use Corning Gorilla Glass to avoid scratches. I wonder though if there's a matte version of that though for the e-ink side for outdoor reading, if e-ink screens use glass at all (I'm not certain). There are matte screen protectors though I don't know if they'd be more problematic since they might get scratched up which isn't wanted in the first place.
Anyway, this potential product could satisfy some, if not a lot of people, who have/want both an e-ink reader and a normal tablet.