I've not seen much in the way of third party combat traits - but I like the idea for two-weapon fighting to leverage this. Something like "When you are wielding two non-shield weapons that are supported by the combat style, you roll two dice and take the better result when determining initiative" perhaps?
I do agree that they aren't usual for heavy battlefield usage, but things like sword-and-dagger styles were perfectly functional during the era of light armor on non-battlefield situations.
I believe at least one previous edition of Runequest had a "parrying dagger" which was more expensive than a normal dagger (and may have done 1 less point of damage? Or couldn't be thrown as well? not sure), but counted as Medium instead of Small for parries.
RQ6 has a Main Gauche which is entrapping. So, using this instead of a normal dagger is already a good step along the way.
One final suggestion for an alternate combat style trait: Allowing a short sword in each hand, as an alternative to the rapier/dagger combo. It increases the size and damage of your off-hand weapon, at the cost of lower damage on the main hand.
They don't work well as mass combat weapons and I'm not going to argue that they're in any way superior to the larger shields, but I'm really unconvinced the "you're better off with a single weapon" argument is true, because personal experience seems to suggest otherwise.