Anyone who has shot a pistol knows that you need to aim it to be accurate, which requires you to look down the barrel. If you are shooting two guns, you arent aiming down either. Both should get a penalty.
When I said the rules cover using a weapon in each hand I think I misspoke. What they actually cover is one-handed shooting (strangely, no mention in the rules of using melee weapons one or two handed). As there is no penalty for shooting a pistol one-handed I naturally started to wonder about using a pistol in each hand.
There are already rules for aiming. If you don't take a fast "Aim" action, the shot incurs a "Quick Shot" penalty (-1 modifier for pistols). However, you raise an interesting point. Clearly it's not possible to aim two pistols simultaneously. I would therefore imagine in most situations in which a character is firing a pistol from each hand, they cannot use a fast "Aim" action first. This would result in a -1 modifier for the pistol in the dominant hand and a -2 modifier for the pistol in the non-dominant hand.
To be honest, I think I've opened a can of worms, as the next logical question would be, is shooting two weapons at once a single slow "Shoot" action? I would think it would have to be, otherwise duel-wielding of firearms would be impossible in the game. The exception would be shooting from the right hand in one turn and the left hand in the next turn (and allowing a fast "Aim" action for both). I appreciate that in 99% of games it's unlikely that characters would be charging around with a gun in each hand but it still might appeal to some players wanting a more "gun fu" style of play.