Of course the JM is better than Wu's staff at level 10, the damage cap is preventing you from benefiting from a weapon damage of more than 10. Once you hit level 20, Wu's will be better for a while since your dual-wield skill will be so low that your off-hand rarely swings, maybe all the way up to your mid-30s.
Keep in mind that Wu's is a level 40ish item while JM is a level 50+ item, so it's more logical to compare JM to IFS or PB. Those two staves will probably be better than dual-wielding until level 50+, and then dual-wield would only be better if you have your epic or AC/SoS.
The reason 2handers are better until late levels is because dual-wield needs a high damage bonus in order to be worthwhile, and also because your off-hand swings don't trigger consistently until upwards of level 40. Even then, you'd probably need weapons with a better ratio than 1:2 because at higher levels you'll get triple attack which, I believe, can only trigger from your main hand and thus benefits 2handers more than dual-wielding.
If you compare two 1handers with a 1:2 ratio (JM, KD etc.) to a 2hander with a 1:1 ratio (which IFS/RFS/PB/T-staff almost have) at the lower levels where you haven't got a damage bonus yet, then you would have to succeed your dual-wield check on every single swing in order for them to even be equal. You obviously don't, so 2handers are better for a long time.
For all of these reasons, classes that can use 2handers tend to do more DPS that way than with dual-wield until about level 45 when the damage bonus has grown big enough to make a noticeable difference when you use a fast 1handers. This is true even for warriors and rangers, and moreso for monks who have access to much better 2handers. I probably wouldn't ever use weapons like JM or KD if I could afford an IFS, I'd want the 15/25 and 17/28 from AC and SoS before I'd care to dual-wield.
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