Minecraft wiki entry for Dual_wield, associated text: The off-hand can use the item it holds when there are no usable items on the main hand (e.g. swords), or the item held on the main hand cannot be used directly (e.g. shovel or hoe with no grass nearby). Otherwise, the main hand takes priority over the off-hand. Players also cannot break blocks or attack with the off-hand, even if nothing is held in the main hand.