IIRC the game does remember the off hand weapon you have equipped. So if you equip two daggers and hotkey the right hand one, then switch to bow, then press the right hand dagger hotkey, it should also put the other dagger in the left hand.
I'd still love a dual wielding hotkey though (hoping for a SKSE version), as equipping a torch for instance will remove the left hand weapon so the vanilla system is far from ideal.
Right now, 30 seconds ago, I tried it. If you equip a spell in the left hand, hitting the hotkey to equip a dagger causes you to UNequip the right hand dagger...and that clears the hotkey. I have "steel dagger (fine)" set for #1. "Healing" set for number 4. If both hands are empty, hitting #1 will equip both daggers. The first time I hit #4 the left hand will equip the healing spell. If I hit #1 again to re-equip the dagger, it will instead un-equip the right hand dagger, and after that #1 won't do anything. It clears the function. I hit "Q" to see the favorites listed and "1" is no longer set beside anything, when previously it was next to "steel Dagger (fine)".
I want 1 button that will equip 2 daggers at the same time. Then when I hit the key to equip a spell, I can just hit the dagger key again and it will revert to me holding a dagger in each hand.