As of snapshot 15w31a, you can hold things in both of your hands.
I figured out that F lets you hold the item in your other hand. But how do I use this item? Can it even be used?
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Sign up to join this communityThe item in your offhand can only be used for it's right click function (i.e. eating, placing blocks, blocking with shields).
Note that when you right click, the game first tries to use your main hand, and if the item in it does not have a right click function (or that function is inapplicable where you are targeting), checks your offhand. The sword can no longer be used for blocking since 15w33c, so you can use things together with sword and most tools.
For the left click function, only your main hand item can be used.