Ah, no, it's down the bottom left, underneath the melee/ranged switch, and colured in a light grey that makes it easy for the eyes to gloss over without registering. It's a little left-right toggle between "One Handed" on the left, and "Dual Wield" on the right. Oh, and I believe you'll need to check and reset it every time you change your melee weapons, as well.
