Two handed offers more damage than using a shield, however, also less damage than dual wielding, while granting no defensive bonuses. Let's not forget that, aside from spears, you only get a .3m range increase over one handed weapons. There's also the issue that skills like Sparkstriker scale twice as well with dual wielding due to the extra attack, when compared with two handed weapons.
Actually Two handed gives more damage than dual wield - see details here:
https://www.reddit.com/r/DivinityOriginalSin/comments/70nix2/1h_vs_2h_the_crunched_numbers/Thats based on the raw numbers without considering skills and abilities. If you calculate them in two handed is better on the ability side (because +5% crit multiplier) and dual wield can be better on the skill side (ex. Sparkstriker). I think they are already rather balanced.
