Ah! This is what I mean when I speak about game not being tactical, like Pathfinder vs Savage Worlds, which is.The concept of flanking/ganging-up seems like it comes through other action synergies, rather than a specific rule for it. One person knocks an oppent over, the other gets D6 extra damage on them. On person uses up their parry/evade, the other whollops em. One person has backstab, and just... backstabs em.
I don't think there needs to be a bespoke rule to represent outnumbering an opponent, because all that comes through pretty clearly with the existing mechanics.
I'm not saying there are gaming decisions in Pathfinder, but they equate to picking up bonuses for your die by preset actions and conditions relying on character build. In Savage Worlds, you use game rule available to everyone to create teamwork and use the terrain to your advantage. Those do gather bonuses but more based on using cover and outcomes of the fight.