I don't know if this is right ... but IMHO rings/amulets with something like 2-4 dmg apply to BOTH weapons when DW'ing. They have the same effect red gems in weapons have.
Those rings/amulets are OP anyway. 2-4 dmg is worth what ... 60 dex?
Edit: a 5-10 ring with a 22 dex gem gives me 120 dmg ...
Yeah, a lot of +Damage on equipment greatly skews things. I probably should have thought of that before I made the blanket statement :x But, yes, all your (non-Weapon) +Damage applies to both Weapons. You can get enough +Damage that more than makes up for the listed damage discrepancy between your two weapons.
I'm not sure how well that scales into, say, Inferno though. I mean, +30-40 damage is neat but on weapons with nearly 1000 base damage, it's mostly cute.
Basically, Weapon Damage when DWing is:
(MH_AvgDmg + OH_AvgDmg + 2*DmgFromArmor) / (MH_AS + OH_AS)
With MH_AS and OH_AS being the Attack Speeds with the 15% bonus to them. The single wield formula is basically the same just without the Off-Hand variables in it.