Tower Shields: How do they work? Work it like the shield spell - you can move it once per round, on your turn, as a free action, defining which half of the battlefield you orient it against. Opponents that are on the "line of defense" that defines the side of the battlefield you're blocking with the shield would have no more than 50% cover against you (and vice versa). Opponents that are on your side of that line would not be blocked by the shield at all.
Can you Ready an action that lets you duck completely behind the tower shield, giving you 100% cover? Yes, you can. That of course means you're not doing anything but hiding, though.
If flanked, can you pick one opponent to block 100% and one to block 0%? Yup.
Would this tactic negate the flanking bonus? Nope. The second guy is still a threat ... he might break through your shield, or hit you with a touch attack spell, and so on. (Sean Reynolds, 3E board)