The way I read it is that before, you would see a weapon that gave Dagger skill +10 and that would give you accuracy and attack on all both weapons if you were dual wielding.
The new stat will only apply to your main hand.
They also plan on using this in place of + acc or + attack from now on. So instead of making a dagger with 10 acc, 10 attack and 5 skill, they will just make one with 188 skill for no apparent reason :P