How do I use two one-handed items at once?

Right click the item you want to hold with your right hand and left click the item you want to hold with your left hand. Both items must be in your toolbar and they have to be 1-handed items, such as 1-handed swords, axes or flashlights. You can combine any of them freely.

There are two center slots for each of the two mouse buttons. If you drop an item into the right-handed (red) slot and wield a one-handed item from your normal 10 numbered slots, you will wield both items. Furthermore, pressing X will allow you to switch to solely your blue slot, and Z allows you to swap the blue with red, and vice versa.