If you're talking about this concerning monsters then I believe that they will be in packs of a sort and I imagine that tactics like that won't fully work.
And if you're talking about NPC guards for towns thenmaybe once a player from an enemy tride has been seen the guard could run to the tribe totem to raise an alarm? Thus notifying all NPC guards of the hostile players.
Having the ability to stab in the back deeply appeals to my D&D side. Backstabs shouldn't be considered exploiting in my opinion, but it would be interesting to see an NPC aware of more than one enemy.
I can imagine him changing his guard stance to better face both enemies. A system that comes to mind is Assassins Creed's, although that would probably be ridiculous to implement in an online game.