My understanding is that the mainhand and offhand swing completely independently of one another. This means (I think) that, as long as autoattack is on, your offhand has a certain % chance to swing once its weapon's delay is met. This probability is based on your DW skill.
Hence the conventional wisdom regarding how to equip weapons for maximal dual-wielding DPS: your mainhand weapon should almost always be the fastest weapon you can come up with, as it will be the only one of the two affected by your level-based primary-hand damage bonus. Do the math with each weapon, as there are (rare) exceptions to this rule. On the other hand (couldn't resist), since the offhand has no damage bonus, its damage output will be directly proportional to its weapon's ratio. This is why, for monks, the Adamantite Club (ratio 15/25 = 0.6) is better in primary (at 60, the damage bonus makes it 26/25 = 1.04) than the Stave of Shielding (w/ bonus 28/28 = 1.00), but the SoS is superior--disregarding the AC's proc--as an offhand weapon. The SoS's innate, unbonused ratio of 17/28 = ~0.61 trumps the AC's flat 0.6. This is also why the monk's epic fist ((9+11)/16 = 10/8 = 1.25 at 60) trumps everything as a primary-hand DPS weapon.
One of the rare exceptions to this rule would be the efficacy of the ranger's Revultant Whip (5/14) vs. the Fayguard Parrying Dagger (9/17). At 60, the latter's 20/17 = 1.17 > the Whip's 16/14 = 1.14. In the offhand, though, the Parrying Dagger would always outperform the Whip due to its better innate ratio.
Anyway, I know that became a bit more of a digression on the general mechanics of 1-handed DPS than you asked for, but I hope it's helpful (and correct).
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