I don't know if this is possible or not but I thought I'd throw this idea out for you to consider:
An attack vs. attack type of implementation. Your parry as it is now is really just blocking, and it requires its own keybind. What if instead of a parry key, you simply attack the opponent's attack?
For example, The opponent swings his right hand sword: You swing your own right hand sword to connect with his, and the two attacks are compared in some way to see which one wins out. The loser of that comparison is then staggered or penalized in some way.
I think this would be a balanced way to implement dual wield parrying. Because not all of your attacks and parrys will succeed, it's not overpowered, and you still have a way to defend yourself as a dual wielder. To top it all off, good parrying would give an offensive advantage.
Ideas for the attack comparisons include: Swing timing, block/weapon skill, perhaps modifications based on weapon type(harder to parry if opponent has shield, or etc.), and normal or power attack.
What do you guys think?
There are 2 problems imo:
1) The implementation; currently my mod is very simply just activating the vanilla block animation under certain conditions (key press etc), and everything else is handled automatically by the game because it's really just blocking. So it's a pretty simple mod really. What you're suggesting is an entirely different system which, if it's even possible to implement, would probably requires hundreds if not thousands of lines of scripting, and I've personally really got no idea how to do it.
2) I don't think it's entirely realistic and personally I'd prefer it the way it is now. In a real fight I don't think swordfighters defend themselves by hacking and slashing at the opponent's weapons. They raise their weapon to deflect the force of blows and try to hit the enemy (not his weapon) when there's an opening