Before 1.0, you put items in your hotbar, left clicked one item and right clicked the other. These must both be 1-handed items. After 1.0, each slot in your hotbar has a "left" side and a "right" side. Putting 2 1-handed items in 1 slot (one on the left side, one on the right side) and then selecting that slot will result in you holding the 2 1-handed items. These can be any 1-handed items: salves, shields, food, 1-handed swords/guns/spears, but 2-handed items (blocks, some swords/guns/spears) occupy both sides of the slot. You cannot wield a 1-handed item and a 2-handed item, nor can you wield 2 2-handed items.