For those trained in the martial arts (all player-characters, among others), combat always makes use of both hands. Even a bare hand can be used to grab or deflect most weapons under the right conditions. However, if you have a free hand, it’s better to have it wield some sort of useful item than not, whether it’s another weapon, a shield or another parrying item such as a cape, a torch or even a cloth hat. Possible fighting styles include:
Two-handed weapon: No combat bonuses or penalties.
One-handed weapon and buckler: No combat bonuses or penalties per se, though the buckler will confer an additional armor point if you are not wearing plate armor or a breastplate.
One-handed weapon and parrying item: Permissible parrying items include a cape, a leather gauntlet, a torch or a cloth hat. (The latter only counts if opposing a dagger or rapier.) Impermissible parrying items include a lantern or a pistol. Permissible parrying items confer no combat bonuses or penalties.
One-handed weapon and second weapon: If you are wielding an average or larger weapon (3 ft or longer in length) the second weapon cannot be longer than ⅔ the length of the main weapon. Otherwise it can be up to the same length. (Thus you can fight with a backsword and a ballock knife, or two ballock knives but not two backswords.) On any natural roll of 7, the second weapon will do 1d3 of damage (whatever the usual damage of the weapon). If this roll was a hit, then the damage will be in addition to the damage of the main weapon.
Case of rapiers: Fighting with two rapiers gives you a penalty of -2 to hit but improves your armor class by 2.
One-handed weapon only: If none of the above conditions apply, but your opponent does satisfy one of them, then he will be at +1 to hit. The reverse also applies, of course.