It has always been frustrating not being able to wield projectile weapons with off-hand weapons or, more generally, to quickly change between various types of weapons. IWD2 solved this by enabling weapon sets. In BG, one can always swap weapons and/or shield sets for others via inventory, but it's tedious.
I wrote a script that does this job as quickly as can be done upon a simple key press. It requires AutoHotkey. Open the script with a text editor and follow the guidelines (preceded by ";"). I suggest you use Notepad++ and format the script using Language>A>AutoIt in the editor to have a better view.
It's not a mod per se but I think it belongs in this section since it requires some customisation on your end. It's a rough job that can be improved if people actually need it.