Currently I can find no perceivable reason to use two-handed weapons over one-handed weapons, so I have made a list of the disadvantages and suggestions for rebalancing.
1. You can potentially have twice as many enchantments by dual wielded one-handed weapons, so enchantments on two-handed weapons should be twice as powerful.
2. You can do significantly more damage by hitting with two one-handed weapons simultaneously, so two-handed weapons should have significantly greater damage range.
3. Dual wielding effectively doubles your chance to hit and parry, while some two-handed weapons don't even get a bonus to hit/parry. All two handed weapons should have significant enough bonuses to hit/parry to even the odds.
4. Even with these tweaks, I'm not sure you would be able to match the damage output, parry and hit probabilities of dual wielding because of how these things are calculated, but something that could be done to give two-handed weapons a unique and sensible advantage is by increasing their reach by 1 tile.
EDIT: Because adding +Hit/Parry bonuses wouldn't actually balance out the probabilities of a two-handed vs dual wielding due to how hits/parries are calculated, could two-handed weapons be given a double roll for hit/parry probability?