Dueling fighting style requires you to only wield one weapon at hand. You can have a shield at the other but not a second weapon, nor you can hold a versatile weapon with both hands. Versatile weapons do greater damage if you do hold them with both hands. So they should not
work together while using both hands with a single weapon.
As with Soulstalker's correction, you can also gain both benefits of "Dueling" and "Great Weapon Fighting" if you are wielding a versatile weapon with one hand.
Can you check your combat log to see if +2 damage roll is added?
- Because if it's added, it's a BUG.
- If it's not added, we still need a way to use "one hand only" to get benefit from dueling when we want. It's a FEATURE missing.
I can also check this when i play late evening