if you already have a good OH with stats and the DPS that you want. I'd recommend keeping it around already.
The major difference is if you have two weapons of the same speed. Where you get that phantom extra flurry tick.
The second major difference is whirlwind dps.
Since ww is normalized, your ap input into the equation will be the same no matter what speed weapon your useing;
Weapon damage + (AP * 2.4 / 14)
So when you increase weapon damage by a factor, your ww's will be doing that much more...
Ok, so from what I know, if both weapons hit. The damage will be increased, no matter what, since your AP is generally a more significant boost to weapon damage than the weapon itself (for D/w at least). Just having the OH hit at all will make a big difference in DPS.
But if you want to min/max (which is of course ideal). I'd say two same speed, slow/slow weapons will be the most you can squeeze out of WW.
As far as I know, the new ww formula will look like;
5% miss chance;
MH Weapon damage + (AP * 2.4 / 14) +
50% * (OH Weapon damage + (AP * 2.4 / 14))
-OR-
62.5% * (OH damage + (AP * 2.4 / 14)).
So if you increase your weapon damage from 100-200 to 200-300, you'll see a 100 weapon damage increase, or 50-62.5 damage increase per WW.
WW on a 9 second rotation, 50 damage increase = 5.5 dps increase.
WW on a 9 second, 62.5 damage increase = 7 dps increase.
From going from a 1.5 speed OH to a 2.6 speed OH. 97.7 dps base.