I thought it worked according to a mithral breastplate being a light armour...
could you vouch that since they give -3 Armour check penalty...
No, you did not waste your time. You are absolutely free to make a house rule that mithril weapons reduce the penalties for two-weapon fighting. You could even drop the penalties for two-weapon fighting altogether, if you wish.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.