This should be a simple enough addition. A dagger in the offhand is a traditional weapon choice based on real-life use, and is pretty cool RP-wise as well. Unfortunately, unless I'm mistaken, there aren't any daggers that really lend themselves to being dual-wielded currently in the BG series. Sure, they can be effective in the offhand, but none have the power of wielding Crom Faeyr, Belm, or the Equalizer in a character's off hand.
What I'd like added is simple: A handful of new magical daggers that grant either AC bonuses (Parrying daggers) or attack bonuses like the aforementioned notable weapons. A dagger that's literally just a copy of Belm but smaller would be a good start.
Here's an example from IWD 2 (Obviously the stats are 3E and don't translate, but you get the idea):
"Baron" Sulo's Hook
Damage: 1d4 + 1
Attack Bonus: -1 penalty
Damage Type: Piercing
Deflection Bonus: +3
Saving Throw: +1 bonus
Resistance: 5/- Magic Resistance
Weight: 1 lb.
Feat Requirement: Simple Weapon, Small Blade