Just a comment from an 'ex' medieval weapons fighter ...
I have long since given up my Dark Ages 'sword fighting' (And axe, spear, quarterstaff, dagger, etc!), but I think that the penalties for a Trained Two Weapon fighter should not be so severe as -6 (Offhand).
When fighting with two weapons in my 'heyday', I found it quite difficult to make two separate hits, coordinating both weapons simultaneously - when they were both long handled (Full length, such as axe/ sword, two swords,etc). When I used parrying dagger and sword, I was fine (Zero penalty) - and used the parrying weapon (Either!) quite effectively, giving me a strike with either 'free' weapon.
I know quite a few experienced 'Living History' fighters who are brilliant at full two Weapon fighting, and can be quite scary to see (And Fight!). They were just as effective with either weapon. Maybe this was their 'natural coordination (?) - so maybe High Dexterity counts, as well as experience/ training.
Untrained ... Two Weapon combatants should recieve a hefty penalty, because it takes quite a time to perfect the fighting technique.
Does Anyone have anything to offer in addition?