There is no cap on dual wield as far as I know. There is, however, a delay reduction cap of 80%. For example, if you have 50% dual wield, and 40% haste, you have (1-.50)*(1-.40) = 0.3 delay multiplier, which is a 70% delay reduction. Anything beyond 60% haste would have no effect, unless you removed some dual wield gear.
XI Wiki


