Yes you can. You can block nados with claws.
Block from a shield is useless in CvC (Caster versus Caster) in this case Druid versus Druid. So your 100 points investment (20 levels) in dex has no use. Eventhough you come up with greater damage and higher life you'll go into fhr animation from his faster nados, so to say, you'll have to play very defensive to win. I guess it'd go like this: You both casts, you both hit, you both go into fhr animation, but his attack hits a bit earlier since he casted a bit faster, so sooner or later he should win.
To be precise it would go like this if you are equally skilled:
Both cast Tornado, one with 25/10 or 2.5 attack per second, the other with 25/11 or 2,27 attack/second. Let's say you've teleportet on top of eachother so the range between you will be 0 yards.
As already mentioned you'll hit the mouse button on the same time. I guess you both have 50% pdr then the setup would be:
2,27 attacks per second: life = 5300: damage 6600/12 = 550
2,5 attacks per second: life = 6000: damage 5500/12 = 458
FHR = 6 frames for both, 6/25 second = 0,24 second.
If the range is 0 tornado should hit in the moment you attack, so: the druid with 2,5 attacks per second should hit first, and win no matter what due to the fact that you goes into fhr animation. You'll only get 1 attack off out of every 0,4/0,24 = 1,6 attemps. So it's 2,27/1,6 = 1,36 attacks per second
1,36 * 550 = 750 damage / second
2,5 * 458 = 1145 damage / second
5300 / 1145 = ~5 seconds for victory (less actually)
6000 / 750 = 8 seconds for victory
So it's easy to see that the druid with the fastest cast rate will win this one. Now if the range increased and if Tornado travles slow enough the picture could change of course. (Didn't include Hurricane's damage since I don't know their cold resist, +% damage to cold or -% cold resist).
Personal comment:
I can't see why DvD druids just don't try to achieve higher life than the opponents damage, with 50% damage reduce the damage have to be 12/12 of the life to stun lock, or so to say: damage >= life before "stun lock".
With high level oak sage, a good BO, max vita, lots of + life aso. you won't even need those 99 fhr.

E.g. I've heard about Druids with 8k-9k of life without being transformed, but the highest Tornado damage I've heard about was about 7.5k or so, now I've these numbers from asking people on b.net who could easily exaggerate.