Actually, Stevens probably did have two security of his own, but they fled without finding and bringing him with. One of the security sent from Tripoli to rescue those at the safe house was killed, and the other was already at the safe house and on the room defending the building when a mortar round landed on the roof and took him out. CBS has a nice video report of the safe house showing bloody flak jackets and exploded mortar rounds on the roof.
Was Stevens two man security detail American or Libyan?