That would be pretty cool, I just worry about how to balance it.  But, because it takes a special rare trait to do, I am fine with it being better than the options of 2-handed weapon or sword & board. But it can't be soooo good that you should only want to choose that one.

I agree, adding the choice of wielding pairs of daggers sounds like a cool option for the Assassin path.

I'm also thinking of a ranged ability for this path called "backstab", where the unit would become translucent (invisible for the enemy), walk to the back of his target, hit it with his weapons with a guaranteed crit (from his dex stat plus gear with crit, etc) and then walk back to his starting position becoming opaque again.

A counter for that, if you see rogues/assassins on the other side and you want to protect a weak unit, would be to place a stronger unit behind the unit you want to protect, so as to act as sort of a bodyguard. As long as the weak unit has someone on his back, it wouldn't be possible for the assassin to target him with this skill.

Or just make the second weapon only do half damage.

The concept is cool and I'm positive there would be a way of balancing it such that it was strong (because it is a rare trait after all and it would need to be better than an average trait in order to make it worth picking) but not overpowered.