Even under the most advantageous conditions, two-weapon fighting (TWF) under AD&D rules suffer a -2 penalty to the ability to hit an opponent with an attack. They have either not found – or more likely they have totally ignored – the recognised schools of two-weapon fighting, such as the Renaissance style using a rapier and a dagger. How on earth did they imagine the Main Gauche dagger got its name?
OK, accepting that the left-hand weapon cannot strike as easily or as often as the one in the dominant hand and is used in part for parrying, there is no reason for the weapon in the dominant hand to suffer any attack penalties if used in a recognised TWF style. I've not been through the calculations that would be required to work out what would be needed so that the game woould not be unbalanced, but it would probably be playable (and more realistic) to limited the style to light weapons only with a dagger in the off hand, leave the dominant weapon unpenalised, and either give the dagger a -3 hit penalty with no armour bonus (from parrying) or -4 hit penalty and +1 armour bonus, or possibly a -5 hit penalty and +2 armour bonus.