Zweihander does do "double" damage - if you put two 6s into it, it deals 2x (6) damage. This is consistent with other doubles items - Sneeze, Lunasa, Skeleton Key, Memorise - they all take doubles but only refer to a single dice.
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.
