Well I have been using a shield in dark fathers and havent noticed a difference at all. None, zero. Counter attack is still going off left and right. Which means a successful parry. I think it takes the highest, or each gets its own chance, the 1h weapon gets 37% and 5% for the shield. Its is honestly not making an improvement and its not worse by any means. I would notice against a dark father.
Gonna do more testing later, but it seems the info on stratics is wrong, or worded wrong. I am pretty sure it goes like this...
The high you have in both skills, the more your weapon has the chance to parry over your shield and thus your shield loses its chance to parry. So you can go without a shield with a 1 hander or two hander with a higher percentage. But having the shield equipped will not effect the percentage of your one hand weapon to parry a blow.
All it states on stratics is that the higher the skill you will lose the chance to parry with a shield. It dont outright say that using a shield with a one hander that the shield will nullify the 1 hand weapon. Just that your shield wont parry. And my current tests and proving my theory to be correct. Your one hand weapon still parrys.
A better way to test it would be unarmed females in jhelom with a fully repaired one hander and shield. If the one hander loses durability it proves my theory to be correct. Mind you, you are not attacking. I just trained parry on 3 toons and all of them lost durability on weapons cause they all had bush, so I used a rune blade. Started with 100 durability and it went down at a steady rate. So that was a 2 hander, now if I test with 1 hander and shield and 1 hander loses durability, while shield is equipped, it just moves parry chance from shield over to weapon. If both lose durability, its parrying with both. If only shield loses durability, then 1 hander is not parrying at all. But, I am willling to bet both will lose durability and thus will put my theory to the correct realm.

More tests later today.