One of the main benefits to carrying a weapon in each hand, apart from looking cool, is that it gives you more options in combat. Even if you only use one weapon at a time, carrying two saves you the effort (i.e., actions) required to stow one and ready another. Load one Predator with non-lethal ammo and one with explosive or armor-piercing, and you have choices. Carry a machine pistol in your off-hand for suppression fire, or a sword to slash at enemies who get too close. Remember that whichever weapon is in your off hand takes a -2 penalty (SR5 p.178) unless you have the Ambidextrous quality (p.71).
Attacking with two weapons simultaneously, John Woo style, means splitting your dice pool between two (or more) attacks. The Multiple Attacks free action (p.164) has to be combined with an appropriate attack action (e.g., Fire Weapon, Throw Weapon, Melee Attack, Cast Spell). It doesn't say you can combine different attack actions, only that you can divide one among multiple targets.
In the odd case where the dice pool would involve different skills, such as making a multiple Melee Attack with a blade in one hand and a club in the other, or a multiple Fire Weapon attack with a heavy pistol (Pistols skill) in one hand and a submachine gun (Automatics skill) in the other, I don't see any clear direction, so it's up to the GM. A friendly GM might let you average the two skill ratings, while a strict one might tell you to use the lowest skill rating.