I don\'t remember whether it was D&D 3rd edition or Midgard 2nd/3rd edition, it was a D20 system in any case (And yes, D20 systems were there before \"Wizards Of The Coast\" \"invented\" it .. anyway, not going to rant...)
Anyway, it does have a skill called \"Two-weapon-fighting\" and a skill (or feat respectively) called \"Ambidexterity/Ambidextrous\". The penalty system is balanced, depending on which of the skills you have. Ambidexterity means that you can fight equally well with either of your hands but doesn\'t mean you can wield to weapons at the same time. So it went something like this:
Standard fighting: +/- 0
Standard fighting (with the off hand): -4
Standard fighting (with the off hand + Ambidexterity): +/- 0 (since there is no \"off hand\".
Two weapon fighting: -6 / -10 (\"normal\" hand / off hand)
Two weapon fighting (with Ambidexterity): -6 / -6
Two weapon fighting (with Two-Weapon-Fighting): -2 / -6
Two weapon fighting (with Two-Weapon-Fighting + Ambidexterity): -2 / -2
You can easily do the math (ambidexterity emilinates a 4 point penalty for the \"off hand\", two-weapon-fighting emilinates 4 points for each hand, since it reflects your ability to combat with two weapons).
I always liked the 2-folded way this was made up, and also that no matter how good you were you still get a minor penalty. Also, now after I\'ve written it all down, the values aren\'t really the correct ones you also could get an advantage/disadvantage when the weapon in your off hand was a small one (like a dagger or short sword).
I\'m not saying PS should have such a system as well, I merely meant to write down how others solved this, perhaps someone has an idea derived from that, or even improving it.