The Crossbow Expert feat states that as a bonus action, you can "attack with a loaded hand crossbow you are holding."
However, nowhere in the rules does it specify a "loaded" versus "unloaded" state for any weapon. There is a "Loading" property (PHB p.147) that just says that "because of the time to load this weapon, you can fire only one piece of ammunition from it…" but doesn't specify an amount of time, type of action, or anything to load the weapon. (And, in any case, if you have Crossbow Expert you can ignore the "Loading" property.)
Under "Ammunition" (PHB p.146) it just says that "drawing the ammunition from a quiver, case, or other container is part of the attack" which seems to imply that crossbows are not kept loaded all the time, but are loaded and fired as part of the same attack. So is there some other action that needs to be performed to load it (so you can have it loaded first, before the trigger hits?)
[The specific situation that I have in mind was the case of a character with Crssbow Expert and two hand crossbows, one in each hand: Can he fire both crossbows every turn, using his normal and bonus actions respectively, even though he doesn't have a hand free to load the crossbows? Do you need a hand free to load a crossbow?]
Best Answer
You load as part of any attack with a crossbow. You can draw the ammunition as part of the attack, so loading it is implied to be part of the attack as well. "Loaded" in this case means only that you have ammunition available for the attack. Per the official errata, "loading a one-handed weapon requires a free hand."
This means that, post-errata, using rules-as-written (RAW), you cannot indefinitely fire two hand crossbows in the same turn. If you have a single attack, you can fire once with your action and once with the second crossbow as your bonus action with Crossbow Expert on the first turn of combat if they start loaded. However, you can't keep dual-wielding because you won't be able to reload with both hands full.
If you stow the second crossbow on the first turn with your free object interaction, you can fire the main crossbow twice on the second turn, since Crossbow Expert does not say the bonus hand crossbow needs to be different than the one-handed weapon that triggers its bonus attack. You may need to load it before the bonus attack using your object interaction, depending on your GM's interpretation of the feat. You can do this on each subsequent turn to fire a single hand crossbow twice per turn (or more than that, if you have Extra Attack).
You didn't request a rules-as-written answer, so I'll note that there is nothing game-breaking about allowing dual-wielding hand crossbows using Crossbow Expert.
Because of the way crossbows work, I think your GM would be perfectly justified in waiving the errata rule in this case but requiring you to get a customized crossbow and ammo belt to allow you to load it one-handed. That would be part of your house rules, though.