Dual Wield is a trait only NIN is able to use. If you sub NIN you are also able to Dual Wield 2 handed weapons. When you wield 2 weapons the delay of your attack is the 'combined' delay's of both of the weapons equiped. NIN has a Job Trait called Dual Wield I, II, III and IV. At each of these mile stones for NIN 5% of the combined delay is subtracted for faster attack. This means if you are NIN main, and you are lvl 75 with just the job traits you will have a total of 20% taken off your combined weapon delay, if you are a NIN at 75 with NIN AF1 Body you will have 25% off. As for this belt it is only active when YOU SUB NIN, it is not active as a main job of NIN. This can be very helpful for jobs such as WAR/NIN or THF/NIN for the 5% off the combined delay. The first Dual Wield JT is active for NIN at lv 10, second at lv 25, third at 45, and forth and last at 65. This means at lvl 30 (lvl of the belt) and with NIN as your sub you will have level 1 Dual Wield Job Trait active, stacked with this belt your Attack delay will be minus 10%, the max you can cut this delay is 15% after you hit lvl 50. Very sexy IMO if you are able to still connect without having a Life Belt (and most of us can), I personally will be carrying both after I get lvl 50WAR/NIN so that if I am not connecting to well I can throw in that extra 10 ACC, but if I can connect I wanna attack faster. :) Hope this helps, happy hunting!
As far as I know you have to have Ninja as sub for the latent ability to work. I think Enhancing the Duel Wield effect lowers the delay on weapons (I think my brother mentioned something about at higher lvls the Duel Wield II, III and so one lower delay)...that's all I can make of it. My Ninja is lvl 25 but I don't even have a Sarashi.