Nice graph - can you do a simple two-dimensional graph plotting the (average/2) of the two weapon speeds against WF benefit, with lines for different crit rates?
I think you'll end up with a situation exactly mirroring the discontinuities in the single-weapon situation. That is, two weapons at 3s speed is effectively the same as one weapon at 1.5s speed. Two weapons at 2.4s speed is the same as one weapon at 1.2s speed. A 2.6 weapon and a 1.4 weapon is the same as one 1s weapon.
Reasoning is quite simple: with two 3s weapons you get 2 hits every 3 second - one every 1.5 seconds on average. Unless your weapons are exactly the same speed, they'll be going in and out of phase with each other, so the effect of syncronisation should be minimal. Even if they're exactly the same speed, I don't think it'll have much bearing on the final outcome. All that matters is how many hits are ineligible due to the internal cooldown.
So actually, I think the first "stepped" graph will be a more or less accurate rendition of WF effects.
To aid debugging your code, here's where you expect to see the discontinuities. I've listed all the "jumps" that fall within the range [0.5, 4]. Obviously you'd have to run the simulator for many hours to see them all clearly. As it stands, everything below about 1.5s is getting blurred out due to statistical fluctuations and/or the resolution of your ticker.
WITH NO CRIT/FLURRY
UNDER THE INFLUENCE OF FLURRY
I'm taking this set of discontinuities as correct within the parameters of the model, since Draghkar says they accurately match what he gets.
If I'm right in the above post, and that you can model dual-wielding as a single weapon of half the speed, then it actually looks as though the effect of weapon speed on DW shaman itemisation should be less than people fear.
Two 2.9 speed weapons gives you 2 attacks every 2.9 seconds, or 1 every 1.45 seconds on average. Two 2.0 speed weapons gives you 2 attacks every 2 seconds, or 1 every 1 second on average.
If you look at the list of transition points above, there's only one "bump" in between 1s and 1.45s. So the difference in WF benefit when going from two 2.9s weapons to two 2.0s weapons is not huge. The relevant transition is the one at 1.3 seconds. Note also that this is one of the "secondary" transitions caused by flurry uptime, and thus will be a comparatively small bump unless your crit rate is high.
What this says is that if your combined attack speed is slower than 1.3 seconds, you'll get slightly more benefit than if your combined attack speed is below 1.3 seconds. Note that this is the attack speed before considerations of flurry etc. - that's accounted for later in the model.
Dual wielding 2.6 speed weapons should give you approximately the same WF benefit as dual-wielding 2.9 speed weapons. Going faster than 2.6s will lower the benefit of WF, unless the other weapon is correspondingly slower: for instance a 2.8s and a 2.4s weapon should work out OK.
We can see this beautifully on that colour graph Draghkar made. The only part of that we're really interested in is the part from (2.0 , 2.0) to (2.9 , 2.9). That graph was made with only 10% crit rate, so the transition at a net attack speed of 1.3s is slight. As you can see, the colour for dual wielding 2.9s weapons is very similar to the colour for dual wielding 2.6 speed weapons, or even dual 2.0 speed weapons.
What would really help is some empirical data to compare this against. Do we have any parses from DW shammies autoattacking against BL mobs with different speed weapons?
Oof, ignore the above two posts - I thought WF had the same mechanics as sword spec in that OH WF procs would trigger MH hits. Since that's not the case, you'll of course get much worse results if you have a fast OH constantly resetting the cooldown and preventing MH procs.