How do you equip dual weapons via a hotkey in WoW?

You need a macro to equip the weapons to a specific slot. You didn't provide the names of your daggers, so this is the best I can offer:

/equipslot 16 *name of main hand dagger*
/equipslot 17 *name of off hand dagger*

Source: WoWpedia.org

You don't actually need to use both lines in a single macro -- you could have one macro to switch off-hand daggers, and another one to switch main-hand daggers, but switching them at the same times has the benefit of not resetting your swing timer twice (which swapping weapons does. No problems if you're swapping while not in combat)

You want to use an equip-manager especially one that is going you if you become weaponless due to wanting to swap an item not in your bags or leave you wish a fishing pole and so forth.

Remember that swapping weapons / off hands leaves you with timer resets, basically weaponless for a brief period of time.