I think Steve will come around to double shields hopefully after this post is done.

First of all the idea of having large shields equipped all the time might seem kind of strang at first.
But let say two large sheilds would be only be equipped when blocking a huge attack from a boss or pulled to much.
Now a large shield and small shield likea small buckler could be used. Just small shield would take the place of a hammer or mace so would act like a main hand weapon.
So there could also equip two small shields (main hand and offhand weapons) No different than any other blunt weapon.
Two abilities that could be in game would be.
Sheild Barrage That is when a tank with two small shields started punching and bashing a target for let say 3 seconds for like 7 hits.
Shield Storm (no reference to WoWs ability bladestorm of course)

But that would be when a tank starts going in a circle and bashes every one in range with his shield.
Sheild Throw (not really double shields just animation) This is were you have a shield in off hand weapon in main hand.
and you have a shield slung to your back that is circular in design that would be thrown. Could actaully be a shield of light for pallies that get thrown that appears on the toons back ready for the toon to reach back and throw it.
So a toon could have a shield in both hands plus a shield on their back back. Yes that is right you heard it here first.......
TRIPLE SHIELDS!!!!!!!. The toon would just temporily put the main weapon which could be a small sheild on his back during the time it takes for the shield to hit all it targtets and come back.