(Well, I actually got this idea from Skyrim

The only problem is that you can get very strong with that fighting style and everybody would equip 2 light weapons immediately (2 times one handed Shadowfang=80 AC vs a two handed Balanced Steelsword=32 AC).
Maybe you learn that fighting style as a reward of a quest or you need a skill:
Assasin:
Stage 1: can use weapons with AC less than 15%
Stage 2: can use weapons with AC less than 25%
Stage 3: can use weapons with AC more than 25%
...
Looking forward to your thoughts!