Weapon parry is accuracy against accuracy. However, for parrying, this accuracy is multiplied by an individual modifier for each weapon, both for the attacker and the defender. Moreover, this modifier can be different for the chance that your attack is parried and for the chance that you parry.
If there are two weapons, the first one has its individual modifier, and the second one has an averaged modifier.
e si ieu muoir, er mi grans deliuriers