In theory, DR would be doubled as well and any magical effect save they would have to roll twice on, as it is two separate hits, just with only one attack roll. But you are right, this is far more complicated then I thought when I wrote it.
As for the 20/15 instead of 20/10... that was to increase the effectiveness of it, however it did not spell this out.
I blame 4am.
Anyway, lets do this again.
The wording change to Paragon Duel Wield is good, so I think I will leave that at that.
Paragon of war has the issue that it has a LOT to explain. It has to deal with magical weapons save DCs, vorpal blades (a good example where Paragon of war WOULD be a downgrade), what BAB it uses, DR, and the like. So combining rolls doesn't seem easy without a wall of text explaining the ability.
Lets see what I can do
Paragon of war: Paragon of War: You need Paragon Dual Wield or Paragon Proficiency to qualify for this Greater Excellence. The Paragon has gotten so Masterful at combat, that his attacks flow into one another, and it is hard to tell where one ends and the other begins. In addition, the true master knows how someone will block a strike, and attack in such a manner that the next attack will be awkward to block.
The Paragon rolls once vs AC, for every two attacks made. For the purposes of BAB and Crit, this is treated as only 1 attack, meaning that on a 20/15/10/5 BAB array, the attacks use 20/15. However for all other purposes, (DR, Magical effects of a weapon, and the like) this is treated as two separate attacks, as the weapon impacts twice.
Now, why do I make it 20/15, instead of 20/10... well from my impression, the whole idea of the 20/15/10/5 array, is that every following attack is harder to make as it tends to be IRL. In this case, however, the idea is you make an attack in such a manner that one attack naturally lines up into the second, so that the weapon is already on the way to your next strike. Think like throwing a haymaker punch in such a way that it punches the guys head into your other fist, a short sword strike that is already turning to strike at the neck as it cuts the stomach, or an upslash attack on the left side of his body, specifically designed so that when he responds to that (too late) he opens himself up to getting smashed in the head by the same weapon.
Or mechanically, if you want to be so minded... it makes the effect better.
Before I work on Perfected Coordination, I want to know, is that any clearer, or should I try a different approach?