Double Slice from Dual-Weapon Warrior Archetype Dedication:

You lash out at your foe with both weapons. Make two Strikes, one with each of your two melee weapons, each using your current multiple attack penalty. . . . If both attacks hit, combine their damage, and then add any other applicable effects from both weapons. . . . Combine the damage from both Strikes and apply resistances and weaknesses only once.

Dual Thrower:

Whenever a dual-weapon warrior feat allows you to make a melee Strike, you can instead make a ranged Strike with a thrown weapon or a one-handed ranged weapon you are wielding. Any effects from these feats that apply to one-handed melee weapons or melee Strikes also apply to one-handed ranged weapons and ranged Strikes.

Persistent Damage is in both the Damage Section and Condition Section so it counts as both unless otherwise stated. It doubles on a Crit, for example.

In this case: Dual Thrower is combining the Damage from 2 Acid Flasks.