I have been searching everywhere for something like this.
I know you can do this for players, but how do you make it work for NPC's?
(Sorry if this seems like a request.)
The optimal way to control collisions between groups now is Collision Groups. The only difference for making the same group not collide with itself is you want something like this:
PhysicsService:CollisionGroupSetCollidable(npcgroup, npcgroup, false)