Everything I ever ran made no distinction, beyond the usual penalties for multiple actions in a single round. EI: you could fire one blaster pistol twice, or fire 2 once with one in each hand. There were no special rules for ambidexterity, etc. Now, if you want to make some, fine. I'd say for anything less than a heavy weapon, the off-hand weapon is at -5 and leave it at that. Heavy weapons (riffle, carbine, sword) would be -5 for the good hand and -10 for the off-hand. Lightsaber is a light-weapon.