The damage increase approaches 100% as you whirl longer. Only for the first 8 frames is the damage increase 50%, if using percentages to quantify a single uniform weapon damage used on both hands.
After frame 4, both weapons wielded may attack at the same time (contingient on their speed.) Assuming both are 4 frame weapons, both will attack every 4 frames after the first 4 frames.
I don't see range 5 helping you much versus V/T's, or should we now call them T/V's, since it looks like the popular build will be a FOH heavy smiter, using Grief to give good smite damage without spending much in the way of skill points into it, allowing the paladin to have a very powerful FOH+Conviction. I'd be more worried about stacking resists while still doing good damage with good AR, and that inevitably means dual weilding for best effect.
I've been pondering this a bit, and I don't see an easy way to have 150% stacked lightning res in hell along with good PDR, damage, and AR. Best I can come up with, and its kinda expensive and a PITA, is to prebuff fade w/ last wish (+56% res all, 11% PDR) then swap to fortitude armour & silence on second hand (95 - 105% res all.) Lets you use helm and belt for PDR (String or Verdungos and.. BER rockstopper? 40/-15/BER COA?, ORT 08 Gaze? ORT 09 Gaze?), and jewelery for AR (highlords+ravens). The damage wouldn't be as great as Grief/Fort/Beast, but possibly better than an "old" EBOTD/Enigma/Beast setup, although you lose an 85% AR boost from not using Beast and cannot hit as often with the silence for long whirls as you can with beast.
Some mod pls copy my rambling into the "planning" thread?