Well, it's confusing then, because the number on the card that says how much damage it's going to do counts up to twice the total of the dice you put in, so it makes you think if you put in 2 6s, you would deal 24 damage.
Zweihander unupgraded doesn't do double damage. Expose weakness doesn't increase the damage numbers when you hit an enemy, but still boosts the damage. And also, Zweihander plus midnight charm, especially if it's upgraded, is op.