I think you have no reasons to believe the Celts used two-handed weapons.
An article on the Prezerworsk culture:
http://www.limitis.org/documentation/Article010.pdf
A two-handed sword is too encumbering and too expensive. With the method of metallurgy of the Celts and Germans, it's impossible to have a good two-handed sword. The two-handed swords are manufactured for the war only during the XIIIth century (the "Branc d'Arçon"). There are some exceptions, but it's always an exception! The majority of the two-handed swords are manufactured for the "pageantry" ("apparat" in french), the great dacian falx are simply a weapon for the poor warriors as the axe for the Celts and the Germans... it's a weapon which penalize the protection, very risky in fight.