Since the current crop of e-ink devices have used glass substrate screens, a two screen device would be much heavier (ok, hav more mass) than a single screen device (as well as cost almost twice as much.) If it was LCD based, it would consume almost twice as much power requiring either more batteries or producing a shorter viewing time. Since we can read only one screen at a time, a two screen device seems like waste of resources.
Now a three screen device -- one each of e-ink, LCD, and LED -- sounds great.
