As far as I remember multiple arms only gives an extra attack to multi-attack actions, so I'd say no to extra attack for offhand. All in all, 4 Attacks with one + 1 attack with the other hand are quite enough ^^

You would think so.

But in Mark of the Xenos several of the Tyranid critters are specifically set up to use Two-Weapon Wielder!

But in Mark of the Xenos several of the Tyranid critters are specifically set up to use Two-Weapon Wielder!

True, but the Two-Weapon Wielder 'Nids don't have Lightning Attack. Imagine a Carnifex using that - now that would be genuine carnage.

Incidentally, Mark of the Xenos lists both Swift Attack and Lightning Attack among the talents of some baddies, like the Purestrain Genestealer and the Lictor. Does that mean it's up to the GM, or just sloppy editing? I'm planning to use both on Sunday, as the team got a bit overconfident, and I want to unleash something vicious on them, but I don't want to go overboard either.

But in Mark of the Xenos several of the Tyranid critters are specifically set up to use Two-Weapon Wielder!

True, but the Two-Weapon Wielder 'Nids don't have Lightning Attack. Imagine a Carnifex using that - now that would be genuine carnage.

Incidentally, Mark of the Xenos lists both Swift Attack and Lightning Attack among the talents of some baddies, like the Purestrain Genestealer and the Lictor. Does that mean it's up to the GM, or just sloppy editing? I'm planning to use both on Sunday, as the team got a bit overconfident, and I want to unleash something vicious on them, but I don't want to go overboard either.

Swift Attack is just the prerequisite for Lightning Attack, at least in Deathwatch.

Two Weapon Wielder gives 1 additional attack with the "off hand" but with a penalty applied to all attacks made.

When combined with Lightning/Swift Attack and Multiple Arms it still does exactly that.

A creature with Lightning Attack, Multiple Arms, and Two Weapon Wielder (lets just assume it has ambidextrous... the thing has 4 arms for crying out loud), can either:

1: Make 3 attacks with no penaltys.

2: Make 4 attacks with a -10 penalty to all attacks.

So it's just up to the GM whether he wants the guys to make that 1 extra attack. It could sometimes make a difference. Such as fighting a character with reaction+ wall of steel + step aside, or attacking a vehicle or static bunker (which count as unaware and helpless targets in melee, super easy to hit but have decent armor).

However, Multiple Arms does not say that you need TWW to make an extra attack.

(It does say so in BC/OW, but they have a different mechanic for how it works.)