I run SR3 but in general, how would you run this?

Basically SR says, that you cannot buff the same DP with the same source. In these cases, only the highest factor applies. Means: weapon focus force 6 and one force 4, means that you only get 6 dice.

Is that an SR4 ruling? And I presume the extra weapon would (SR3)stack/ (SR4) split dice pool?

You get the bonus dice only to attacks made with that weapon specifically, if i remember correctly.
So if you are not actually attacking with both at the same time, you get only 4 or 6 dice depending on which weapon you use.

Rightyho.
Thinking of the Native Amerindian long knife and tomahawk style for a NPC. So good to check..

Do remember that usually the secondary weapon has to be one size category smaller than the main weapon.

What if both weapons are of small size? Like two knives. I can see if someone wants to use two broadswords, but banning two knives is silly.

Only checked in Arsenal (SR4a), but the section for Two Weapon Melee Combat (page 163) simply limits this to weapons of Reach 0 or 1, and says nothing about the off-hand weapon having to be smaller. And on the subject of the OP, the same page in Arsenal says to split the DP before adding modifiers, and the section on Weapon Foci (SR4a, page 199) does say their bonus is a modifier.

SR3 had a Table for this somewhere actually.
Which weapon can be used with what as primary/secondary.

@KCKitsune:
Oi, i like to play Trolls, i should be able to wield 2 claymores as one handed Weapons if i so chose <.<
But yes, 2 small sized weapons should work i think.

then there's the figure of one gun one sword, switching the favored weapon to your dominant hand in case of odd dice pools. i guess a troll could duel wield shotguns if he isn't moving, right?

Guns are a different Matter altogether.
In SR3, a Troll can more or less comfortably dual wield ANY gun.
Yes, this includes the Minigun and the Panther Assault Cannon.

Shadowrun Third Edition Canon Companion Page 94 and onwards are interesting here.
And yes, a Troll can wield 2x Claymore as well.

The lesser races add a +2TN and -2 Power

Due to how the Recoil works, yes, 2xPAC or 2x Barret Sniper Rifle is VERY easy for a Troll to do.

Technically, anybody can do it, but the loss in power and the added difficulty/TN make it unviable pretty fast.

Yes, Trolls WERE the Combat-Master-Race for anything Close-Combat related and if you wanted to simply do unholy ammounts of damage down range with BIG GUNS in SR3.

How would this work with 3rd ed Ambidextrous rules?

If you have Ambidexterity 6 you actually don't need the off hand skill anymore.

Yes but what happens with the 'off hand' weapon focus bonus dice?

You only get the bonus dice from one focus it seems.

So with 2 Knives it would be:
Skill+Force+off hand skill/2, if i understood this correctly.

which is, actually, in accordance with the general rule that any single test can only ever benefit from one single focus, if i remember that bit of magic correctly too.