Pathfinder: Kingmaker

[Inarius]

I suspect it's "if you are attacking someone who is in melee range with someone other than you." Because how could you determine if someone's being threatened by multiple archers?

Well I can confirm its just two people attacking one target is whats considered flanking. Managed to have my archer and arcane trickster on one target and I was getting spell sneak attacks on it without any melee even remotely close to the enemy.