Two Weapon Combat
Training in Two Weapon Combat (TWC or 2WC) allows a character to swing a weapon in his or her off-hand.
Pros and Cons
The biggest benefit of two weapon combat is the extra swing that the character gets. There are a few things that are different when a weapon is swong in the off-hand, such as roundtime, Attack Strength (AS), and Defensive Strength (DS).
Roundtime
Roundtime when swinging two weapons is combined from the base speed of each weapon. If the off-hand weapon is just as fast as, or faster than the primary hand weapon, there is a reduction in the overall roundtime of the swing.
Attack Strength
The AS of a swing takes into account the character's skill with the weapon itself, and their skill in TWC. 3/5ths of one's skill in the weapon type accounts for the character's AS, while 2/5ths are the character's training in TWC itself. Also, if a character's strength bonus is higher than the character's dexterity bonus, the off-hand attack will be limited by the character's dexterity. More often than not, this comes into play when a character uses the combat maneuver, Surge of Strength.
Additionally, a character that trains in TWC twice per level has no chance of "not finding an opening for the off-hand strike" when swinging at a target up to five levels higher than themselves. Less training reduces the chances of the off-hand strike from occurring. Training in TWC 1.5 times per level reduces the level of the guaranteed strike to like-level with the swinger, and training only once per level reduces the chances to 80% when swinging at a target that is the same level as the swinger.
Defensive Strength
A character's DS when using an off-hand weapon is calculated simularly to the DS granted from their primary weapon training. While the primary weapon grants DS according to the following formula
(Weapon Skill+(STR/4)+(DEX/4)) × (0.20 + stance/2) + (stance × 25) = Parry DS
Off-hand weapon DS is calculated by the following formula:
(TWC Skill+(STR/4)+(DEX/4)) × (0.10 + stance/4) + (5 or 15) = Off-hand Parry DS
Where stance is a number between 0.0 (in offensive) to 1.0 (in defensive) and STR and DEX refers to the bonuses given to the Strength stat and Dexterity stat. The +5 at the end of the formula is +15 instead if the user is wielding either a main gauche or a sai in the off-hand.