The description of the Dueling fighting style (on PHB p. 72, and in other class descriptions) says:
When you are wielding a melee weapon in one hand and no other weapons,
you gain a +2 bonus to damage rolls with that weapon.
The description of the thrown weapon property (PHB p. 147) says:
Thrown. If a weapon has the thrown property, you can throw the weapon
to make a ranged attack. If the weapon is a melee weapon, […]
The new UA, Class Feature Variants, adds a new fighting style option called Thrown Weapon Fighting:
You can draw a weapon that has the thrown property as part of the attack you make with the weapon.
In addition, when you hit with a ranged attack using a thrown weapon, you gain a +1 bonus to the damage roll.
Does the Thrown Weapon Fighting fighting style grant a +1 to the damage of daggers when you throw them one at a time?
In this question, it was answered that thrown weapons can get the +2 damage bonus from the Dueling fighting style. So if I have both the Dueling and Thrown Weapon Fighting fighting style, would I then have a +3 bonus to my damage rolls if I throw one dagger at a time?
Best Answer
It is 100% dependent on whether you think the Dueling Fighting Style applies to thrown weapons; a question already discussed elsewhere
If Dueling Fighting Style doesn't apply to thrown weapons then clearly these never stack as the Thrown Weapons Fighting Style requires a thrown weapon. Below I will show that if the Dueling Fighting Style does apply to thrown weapons then they do stack. Seeing as how the question about whether the Dueling Fighting Style applies to thrown weapons already exists, and answers argue both ways, there's no real reason to answer that question here.
The Dueling Fighting Style states:
The only relevant thing this requires is that we attack with a melee weapon.
And the Thrown Weapon Fighting Style states:
This requires a weapon with the thrown property, and that we make a ranged attack using said weapon. The thrown property shows that a melee weapon, when thrown, is making a ranged attack:
All we need to do then is throw a melee weapon with the thrown property and we would benefit from both Fighting Styles.
Note that these do not add together when using a dart (or technically a net, but that has no damage roll anyhow) as they are ranged weapons with the thrown property, so Dueling can't apply.
I see no reason that the damage bonuses wouldn't stack, the only thing you cannot add twice to a roll (as far as I'm aware) is your proficiency bonus:
A small note on wording:
The Thrown Weapon Fighting Style technically only requires a weapon with the thrown property if you're drawing it. The damage bonus applies to any ranged attack using a thrown weapon. This means that technically it applies to ranged improvised weapon attacks as well:
Note, however, that the Dueling Fighting Style would not apply because improvised weapons do not have the melee or ranged properties, they simply are weapons used to make melee or ranged attacks.