The basic 3e/Pathfinder rules seem designed to encourage the (possibly historically erroneous) companion weapon style sometimes referred to as “Florentine:” A rapier or other long sword in the main hand and a parrying dagger or other small weapon in the off hand. However, the cost of specialization is so great (in addition to the already prohibitive feat cost to get dual wielding anywhere close to using a two-handed weapon out of the box) that, in my experience, all dedicated two-weapon fighters just double up with two of the same weapon (typically the short sword). If you’re going to take feats to improve a weapon, it would take twice as many to improve two different sized blades, and the improvement in using a longsword or rapier in the main hand quickly disappears once focus, specialization, and critical feats are in play. Effectively, the most recognizable dual wielding style becomes the hallmark of an amateur.
I’m currently running Curse of the Crimson Throne and two of my players decided to go the two-weapon fighting route. Since the modules feature a major NPC that teaches what appears to be a dueling style that uses a companion weapon, I figured the systems below were appropriate. These rules don’t work with the Duelist prestige class, due to that class’ restrictions, and I don’t have enough experience with Duelist to know if it’s worthwhile to ease these restrictions. Since a parrying dagger style appears to historically have given way to the empty-off hand style favored by the Duelist, it may make sense to keep the feat chain and the prestige class incompatible.
Companion Weapon Style (Combat)
When armed with a Rapier or Longsword in the main hand and a Short Sword, Dagger, or Cloak (see below) in the off hand, you are treated as having Weapon Focus with both weapons. Both hands must be wielding these weapons and able to attack to gain the benefit of the feat (e.g., if you are disarmed or otherwise lose the use of one weapon, you lose the Focus in the other). You must wield weapons with which you are proficient to gain the benefits of this feat.
Prerequisites: base attack bonus +1, Two-Weapon Fighting.
Benefit: You gain Weapon Focus with both weapons (i.e., a +1 attack bonus).
Special: If you have this feat, you can take any other weapon-specific feats as Companion Weapon Style (e.g., Weapon Specialization: Companion Weapon Style, Improved Critical: Companion Weapon Style, etc.). These feats suffer the same restriction as this feat. You must still meet all other prerequisites to take the feat.
Greater Off Hand Parry (Combat)
When fighting with a weapon pair that qualifies for Companion Weapon Style, you may treat the off hand weapon as if it had the Defending quality: each turn, you may choose to treat some or all of its enhancement bonus as an untyped bonus to AC (losing the bonus when making attacks with the weapon). If your off hand weapon is a Cloak of Resistance, you may use the total resistance bonus from the cloak as AC the same way (you cannot normally make use of a cloak’s magical effects when using it as a weapon).
Prerequisites: Two-Weapon Fighting, Two-Weapon Defense, Companion Weapon Style.
Benefit: You can use your off hand weapon as if it had the Defending quality.
Parrying Dagger (Martial)
This is a dagger designed to provide protection to the hand wielding it, making it better suited for parrying. Characters using Two-Weapon Defense gain an additional +1 AC when using it as an off-hand weapon. It otherwise uses normal statistics for a dagger (but, since it is a Martial weapon, it is more limited in who can wield it with proficiency).
A character with the Companion Weapon Style feat may use a cloak in the off hand. A character wielding an off hand cloak may use it to qualify for Two-Weapon Defense and may use an off hand attack to make the Disarm or Trip combat maneuvers without provoking an Attack of Opportunity. A character wielding an off hand cloak may also make an Unarmed Strike as an off-hand weapon attack (with any bonus from Companion Weapon Style feats), but will provoke an AoO without also having the Improved Unarmed Strike feat.