Rogue (Scout) ability.
Skirmisher - At 8th level, whenever a scout moves more than 10 feet in a round and makes an attack action, the attack deals sneak attack damage as if the target was flat-footed. If the scout makes more than one attack this turn, this ability only applies to the first attack.
Spring-Heeled Reaping feat
While using Spring-Heeled Style, whenever you use Shot on the Run or Spring Attack, you can attack two different creatures at any point in your movement.
Though it says "only applies to the first attack", would it be reasonable as a DM to house-rule that if you move 10 feet between each attack they would both count as a Sneak Attack?
Strictly speaking skirmisher doesn't work with shot on the run or spring attack at all. An attack action isn't the same as attacking something, and spring attack etc. are their own different kind of action.
Is that a reasonable house rule on the other hand - yes. Spring attack, even with spring-heeled reaping and skirmisher used twice over is far from being overpowered.