Karameikos and Beyond
From Combat Options Release 1
This rule allows a character to use a weapon in each hand. The weapon in the primary hand suffers a penalty of -2 to hit, while the weapon in the off-hand is used at a -5 penalty normally. Subtract from this penalty the character’s Dexterity bonus, with a minimum penalty of +0 (so a character with 18 Dexterity does not get a +1 bonus to hit this way). Other melee combat bonuses such as Basic Attack Bonus, Strength bonus, etc. are still applied as normal.
The primary weapon must, obviously, be one-handed, and the secondary must be a dagger, handaxe, or similar very small weapon.
The off-handed weapon normally does not get multiple attacks, even if allowed by specialization. Like a monster using an attack routine (claw-claw-bite) the off-handed weapon attacks at the same time as the primary weapon.
Note: Off-handed weapon attacks (with no primary hand attack) are at -3 penalty, with the Dexterity bonus subtracted as above.
As an option, a character using two-weapon combat may choose to use the off-handed weapon as a defensive item similar to a shield. This must be declared at the start of the round. In this case, no special penalty is applied to the primary weapon, and the off-handed weapon adds +1 to the wielder’s AC value against a single melee attacker per round. If the weapon has a magic weapon bonus, it may be applied, but only the base bonus for those weapons with multiple values.