http://imageshack.us/photo/my-images/849/dualwieldblock.png/
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Here's what I wrote in another thread:
Watch the quakecon demo, Todd blocks 2 hits while dual wielding (parries) when fighting that bandit in the cave. You can hear the metal clang twice and the left sword dips down in the middle of the screen a couple times. I think control wise you just attack after they begin their swing and it plays that parry animation instead of swing animation. So you can't block, but you can parry which is the exact same thing, but it doesn't have "block perks". You can deflect incoming damage.
It makes sense too, naturally someone uses left hand for block (prior elder scrolls games), so Todds reflexes would be tuned to block with left and attack with right.
In other parts when the bandit is NOT attacking, Todds left hand swings the sword differently. So there are two separate left handed animations I see, and the one totally looks like a block/parry animation.
To further my point, look at the official video, in part 1 at 3:28. It looks like a parry to me with the right sword. No blood flies and its a more metallic sound being played at that moment, and the shield is put away. Also Todd takes NO damage on that impact!
To me this looks like you can parry. Why?
Enemies swing at the player, we hear a CLANG sound, the sword moves diagonally in front of the player in a defensive position, AND the player takes no damage. If it's not parry, then wtf is it?
I think Pete said you can't block while dual wielding because block is a whole other perk system. But you can deflect damage (parry) if you swing your sword at the right moment when the enemy is attacking.