When a PC wishes to fight with two weapons simultaneously, they can do so by following the following rules.
Double Attack: The PC can make two separate attacks (A) instead of one by sacrificing their movement action (M) during their turn. Each attack is performed with a different weapon.
Damage Diversification: The damage is distributed between the two weapons, which can be useful for bypassing the resistance of certain creatures. For example, if a creature is resistant to slashing damage but vulnerable to bludgeoning damage, the PC can equip one slashing weapon and one bludgeoning weapon to maximize their effectiveness on at least one of its 2 attacks.
Mastery Difficulty: Fighting with two weapons requires a certain level of mastery. The PC incurs a -2 penalty on attack rolls with each weapon due to the complexity of the action.
Secondary Weapon: During a double attack, the weapon held in the off-hand suffers a -2 penalty to damage.