When you lash out with both weapons, you leave no room for the target to escape your attack.
Archetype
Feat type
Archetype Feat
Level
14
Traits
Archetype
Prerequisites
Dual-Weapon Dedication
When you lash out with both weapons, you leave no room for the target to escape your attack. When you use Double Slice, if you miss with both Strikes, choose one of the two weapons and apply the effects of a hit with that weapon. You can't choose a weapon if your attack roll with that weapon was a critical failure, meaning you still miss entirely if both attack rolls were critical failures.
Source: Pathfinder Player Core 2, p. 196
Bring Dual Onslaught into a Pathfinder 2e game with The DM Screen.