Stacking physical documents is one of the main forms of spatio-temporal organization of information. We present DisplayStacks, a system that enables physical stacking of digital documents via piles of flexible E Ink displays. With a conductive dot pattern sensor attached to the flexible display, we dynamically track the position and orientation of these displays in relation to one another. We introduce mechanisms for interacting with these physical stacks for access and manipulation of information using asymmetric bimanual interactions, such as providing contextual overviews. Initial user experiences indicate a preference for linear overlaps as a stacking configuration.