Rurik said:
Actually I still have the attack do Critical Damage to the target - so it can damage the weapon and the target.
This is getting a little confusing now!
I think I see a flaw in your idea, but to illustrate it I'll change tack and come at it from a statistical direction...
Using your house rule Rurik, assuming we have an average attacker with 50% weapon skill... they have a 5% chance of a critical. Therefor 1 in 20 of their attacks will damage the parrying weapon. (The defender's skill does not matter in this circumstance since an attacking crit will always damage the parrying weapon)
If both warriors are using swords, and the attacker has a standard damage bonus of 1d2, the defending sword will suffer 1d8+1d2 damage... or around 6 points each crit. 4 are absorbed by the sword's AP's leaving 2 points which damage the blade. Since War Swords have 10 hit points, they can survive 5 such criticals before being broken, which on average is about 100 parries.
100 parries is about 33 rounds (assuming 3 comabt actions per round), which at 5 seconds per round comes to 165 seconds, or an average of two and three quarter minutes of parrying before a War Sword will break.
If we decide that the attacker is instead using a greatsword, then the war sword will only survive an average of two criticals. This translates to a survival time of 40 parries = 13 rounds = 65 seconds, or just over a minute of successful defending.
This was of course against a warrior of very mediocre skill. If facing a master swordsman with 100%, the life span of the defender's weapon would be halved. When you start adding damage enhancing magic into the equation, then weapons will be shattered even faster!
Which means as characters become more experienced and start facing tougher foes, their weapon's lifespan will diminish proportionately.
Now it seems to me that no warrior would be very happy if their weapons consistently broke after only a minute or two of use! In fact it would lead to a revolution in fighting style, either switching to missile combat, or the adoption of golf-club bags with spare blades...
With this rule, you'd would never see any family heirlooms, or weapons with a history!
How about changing the occurrence of weapons becoming damaged to a roll of a fumble instead?