Using more than two arms to wield a bigger weapon

Question

I have a character who has four arms. Are there any rules for characters with multiple arms wielding two two-handed weapons or using all of their arms to hold one huge (if the character is medium) weapon?

If it is possible to dual-wield two-handed weapons, would both two-weapon fighting feats and feats that affect two-handed weapons apply to attacks? For example if the character had improved two weapon fighting (ITWF), Two-Weapon Defense (TWD), and Power Attack (PA), would the character get 2 off hand attacks from ITWF, deal 1.5× STR damage from PA, and get +1 AC from TWD?

I would prefer official rules but was unable to find any myself, so I will accept houserules/homebrew as well.

Answer 1

There are no rules regarding over-sized weapons and using more than two arms. Normal over-sized weapon penalties apply.

Inappropriately Sized Weapons: A creature can't make optimum use of a weapon that isn't properly sized for it. A cumulative –2 penalty applies on attack rolls for each size category of difference between the size of its intended wielder and the size of its actual wielder. If the creature isn't proficient with the weapon, a –4 nonproficiency penalty also applies.

The measure of how much effort it takes to use a weapon (whether the weapon is designated as a light, one-handed, or two-handed weapon for a particular wielder) is altered by one step for each size category of difference between the wielder's size and the size of the creature for which the weapon was designed. For example, a Small creature would wield a Medium one-handed weapon as a two-handed weapon. If a weapon's designation would be changed to something other than light, one-handed, or two-handed by this alteration, the creature can't wield the weapon at all.

The rest of this answer assumed multiple greatsword (or other two handed weapons).
You cannot use the Two-Weapon fighting feats if you are using more than two arms. You need to use the Multi-weapon fighting feats. If you were fighting with two greatswords, one of them with be primary, the other offhand. Likewise, if you had sixarms, two for the weapons would be offhand, and so on. You will always have one primary weapon.

Multiweapon Fighting (Combat)
Prerequisites: Dex 13, three or more hands.
Benefit: Penalties for fighting with multiple weapons are reduced by –2 with the primary hand and by –6 with off hands.

Stealing the table from this answer you would make your all your attacks at -4 regardless of if they were off-hand or primary: Table: Multi-Weapon Fighting Penalties

Circumstances             Primary Hand  Off Hand
Normal penalties               –6            –10
Off-hand weapon is light       –4            –8
Multi-Weapon Fighting feat     –4            –4
Off-hand weapon is light and
Multi-Weapon Fighting feat     -2            –2

I have found no rule which changes the damage modifier (*1.5) from high strength.

Likewise, power attack applies to all attacks as described, there are no special rules for using more than one two handed weapon. The primary great sword would get +3 per -1 penalty. The off hand greatsword would get +1.5 per -1 (rounded down, so +1, +3, +4, +6, etc. ) Emphasis all mine.

BONUS HOUSERULE: I would not allow a creature to use a weapon who's category was more than two handed, as per the normal rules. However there are items in 3.5 that required more than two hands to use ( I think it was the scissor sword ) and had better stats. I would also allow a creature with more than two hands to substitue the Multi-WF feat in place of the T-WF for classes with bonus feats, such as ranger.

Have a look at the Gythka and Scissors Sword, which are four handed and six-handed weapons from Savage Species (I know, not pathfinder) on page 45.

Answer 2

Based on the official rules for inappropriately sized weapons, you cannot use a weapon if it requires more than 2 hands. There is no mention of the number of hands the character has in that rule.

Inappropriately Sized Weapons: A creature can't make optimum use of a weapon that isn't properly sized for it. A cumulative –2 penalty applies on attack rolls for each size category of difference between the size of its intended wielder and the size of its actual wielder. If the creature isn't proficient with the weapon, a –4 nonproficiency penalty also applies.

The measure of how much effort it takes to use a weapon (whether the weapon is designated as a light, one-handed, or two-handed weapon for a particular wielder) is altered by one step for each size category of difference between the wielder's size and the size of the creature for which the weapon was designed. For example, a Small creature would wield a Medium one-handed weapon as a two-handed weapon. If a weapon's designation would be changed to something other than light, one-handed, or two-handed by this alteration, the creature can't wield the weapon at all.

Emphasis mine.

For dual-wielding, obviously the "off-hands" weapon would not be light so you'd take the increased penalties for that. I think the general rule for multi-handed characters was that there was one primary hand and all others were secondary. By this logic, the primary weapon gets 1.5x damage and the secondary would get 1x (.5 + .5). This question and its answers fill in a good bit more detail on this exact issue.

Answer 3

Many-handed weapon fighting

Pathfinder has no rules for wielding a single weapon in more than two hands.

In 3.5, however, Savage Species did have rules for three-, four-, and more-handed weapons. Each additional hand used to wield the weapon added another ½ Strength to the weapon’s damage. For example, a marilith using six hands to wield a sword would add 3½× her Strength to her damage.

Weapons had to be specially-designed for this purpose, primarily through extending the handle to allow more hands to fit. The base damage doesn’t change (and indeed, the general size of the weapon aside from the extended handle is the same), though.

Ultimately, this could get silly, since multipliers can be dangerous, but only if you get a lot of arms, which tend to be somewhat difficult to get. The rule works reasonably well in most cases, and if someone does end up getting silly numbers of arms, you can always put a limit on how big handles can be or whatever.

Many-weapon fighting

Pathfinder does, however, have the Multiweapon Fighting feat, which allows you to use more than two arms to wield weapons. You can dual-wield two-handed weapons, but since your “offhand” weapon is not light, it’s a bad idea; you still take pretty large penalties. You can take another feat to alleviate that, but it’s not a great use of a feat.

A better solution is to have a two-handed weapon, and two light weapons (assuming four arms). This allows you to benefit from things that only work for two-handed weapons for that two-hander, while also using the other two arms offensively. Unfortunately, this gets pretty expensive, feat-wise. The four-handed weapon is the better option.