Intro
In an inspirational mood some folks in my regular group set out to rebuild Two-Weapon Fighting for Iron Heroes
The reason for this work was that I have disliked Two-Weapon Fighting as it stands for a very long time. I don't mind that IH is unrealistic to a certain degree and that for example zweihanders work a lot better in d20 than they did historically. I don't mind this because the zweihander style was actually used to some degree of success.

One can predict my major gripe with TWF; fighting and attacking with two weapons at the same time has never actually been done, simple because it does not work at all. The only way that fighting with a weapon in each hand has worked in some degree was with one weapon used for attacking and another for parrying and distracting the opponent. In our rebuild we wanted to incorporate this.

The second gripe I have is that TWF isn't actually any good. You spend a ton of feats, at least one for each attack you can make BAB-wise. And what result does this net you? You get to fight just that little bit worse than the guy with the zweihander and one feat: Power Attack. This is even more so in IH than in D&D, because of damage reduction being so common. So the second goal we had is to get TWF do something useful.

We decided to change Two-Weapon Fighting into 'the flexible option'. A weapon in the off-hand could be used to parry attacks or to distract the opponent, we reasoned. This should be reflected in the new TWF.
(Note: I realise that TWF as it stands is a very good choice for characters who add bonusses to each attack, such as sneak damage. I don't find this enough to keep TWF as it is. We did however try to accommodate for this kind of character with our rebuild.)

Goals
So now we had some design goals we wanted to achieve:
- Make TWF a viable choice for more characters, while keeping it attractive to sneak builds.
- Make TWF the 'flexible style' next to the Zweihander 'power style' and the Sword & Board 'defense style'.
- Include attacks with the off-hand weapon at some point. IH is a heroic game after all and heroes are able to do the impossible; at higher levels.

Proposed rules changes
Combat

Fighting with two weapons (Parry style)
If you wield a second weapon in your off hand, you can use it to distract your opponent or block his attacks. A character wielding a light weapon in his off hand gains either a +1 active bonus to defense, or a +1 bonus to attack rolls. A character wielding a weapon with the parry descriptor in his off hand gains either a +1 active bonus to defense, or a +1 bonus to attack rolls. A character wielding a light weapon with the parry descriptor in his off hand gains either a +2 active bonus to defense, or a +2 bonus to attack rolls. You may switch between these bonuses once per turn as a free action.


Feats

Two weapon fighting (Finesse)
Base mastery:Prerequisite: you must be proficient with both weapons. Neither may be a power weapon
Benefit: Your bonus from your off-hand weapon increases by 1.
Mastery 2: You may use your off-hand weapon to make a feint (using your base attack check or bluff, whichever is higher) as a move action (see 'feint', p.196). You do not gain the off-hand bonus to your attack roll or defense, but instead apply it to your feint roll.
Note: You can do this quickened, as a free action, by taking a -5 to your feint roll.
Special: A character who has both two-weapon fighting 2 and improved feint 2 may feint as a free action with no penalty. There is no additional advantage in having improved feint 9.
Mastery 3: The bonus to active defense or attack rolls from your off-hand weapon increases by 1.
Mastery 4: On a full-round attack, you gain an extra off-hand attack at your highest base attack bonus. All attacks (including the extra one) suffer a -2 penalty.
When using mastery 2, you gain a +2 bonus to your feint check instead of this extra attack
Mastery 5: The bonus to active defense or attack rolls from your off-hand weapon increases by 1.
Mastery 6: When wielding two weapons, you cannot be flanked.
Mastery 7: On a full-round attack, you gain an extra off-hand attack, at a -5 penalty.
When using mastery 2, you gain a +2 bonus to your feint check instead of this extra attack. If you give up your extra attack from mastery 4 as well, the bonus increases to +4.
Mastery 8: The bonus to active defense or attack rolls from your off-hand weapon increases by 1.
Mastery 9: You can make a special rend attack with your two weapons. If you hit an opponent at least once with both weapons, or with both ends of a double weapon, you inflict additional rend damage equal to your primary weapon's damage + your strength bonus.
Mastery 10: You may switch between receiving your off-hand bonus to active defense or to attack rolls twice per turn as a free action.


Weapons (Modifications only)
Style descriptors, add:
Parry A parry weapon is usually a light weapon that can be used well to parry attacks and surprise opponents. While wielding a weapon with the Parry descriptor you enjoy special benefits when it is used as an off-hand weapon. See the section about Two-Weapon Fighting for more details.

Weapon qualities
Change the tables to reflect the following:
Dagger; Sickle; Sai; Rapier, Axe, Double; Sword, Two-bladed;  add the Parry descriptor to all these weapons.

Design notes
Removing the need to take a feat to use two-weapon fighting may seem like radical change it first glance. To ease this intuitive objection I would like to present the following:
In IH there are basically three styles of fighting, namely Sword & Board, Zweihander and Two-Weapon Fighting. Of these styles, TWF is the only one requiring a feat but strangely enough it is at the same time the style which has no obvious advantage. Sword & Board gives a passive bonus to defense and Zweihander lets you add 1.5 times your strength bonus to damage rolls. We wanted to put the TWF style on the same level as the other two styles.

As it stands, TWF can only be used with finesse weapons. This is a point of discussion in our group. It is a step away from the old TWF which allowed either Power or Finesse weapons.

Adding a new weapon descriptor seems like a very easy and modable way to show which weapons are suited as parry weapons and which aren't.