You need to edit the weapon.dat to disable dual wielding.
Give me a list of weapons you don't want to dual wield anymore and upload the edited file.
weapon.datHere is the edited weapon.dat. You no longer dual wield any weapons, but you will still have the upgraded accuracy and stuff when you reach the next weapon level. The only difference now is you no longer dual wield weapons.
Find the Rockstar Games folder.
Go into:
/Rockstar Games/GTA San Andreas/data/
Backup your weapon.dat file.
Replace it with the edited one I provided you.