I'm not familiar with the inventory changes in DKS, but in D2:ED with the Xbox version the off-hand was listed right under the main hand in the equipment list, and you could equip weapons the same as the main hand. On the PC someone did mention having trouble trying to duel wield, as the game was a little touchy when drag and dropping items into the equipment slots, requiring the mouse pointer to be right in the centre of the slot.