I'm trying out this new character that I'm planning to become an assassin like guy... But, problem is, I've always had a bit of trouble with dualwielding with two big weapons because the left-handed one always appears from nothing in your left hand. So I'm wondering how effective it is to be dualwielding with a dagger in the left hand. and do I still get bonus damage or even special kill animations with the dagger in my left hand, since the left hand seems to be a bit ignored by the game?

It is viable, You still get kill animations and as a bonus if you have a sword and dagger combo both sheaths are visible on your body

Power attacks while wielding a dagger and a sword grant 4 attacks.