the problem is not in this. You can create a region which would block out the actor easily, but the problem imo is in pathfinding
when both actors move. Then the path of both or at least one must have been altered. I just recently got an idea how to solve it using following logic.
First you will have to create 2d block region which would encompass the other NPC. This will be via script moving with another actor.
Your actor would have perpetual checking of another actors position (with a script containing endless loop). Then if the position was
in the vicinity, it would call again actor.GoTo(originaldestination) method. This would ensure, that you don't run into anothr actor.
But it's untested (just the concept) and I don't know if waypoints won't make too much mess.
Any ideas anyone?