If you are doing this quest as a party, each member gonna have to pop a new set of NMs before obtaining the Key Item. The cooldown timer seems to be about 4 minutes since the last party member got his KI.
Note: The 'Hide Flap' may have moved to another tent while you attack the NMs but will have re-popped within the same area. If this happens just examine the flap on the new tent to receive the key item. The second 'Hide Flap' spawns when you POP the two NMs, not when you KILL them, so if you have someone along who can take hate after the NMs are popped, it is not necessary to kill them. Also, do not try to examine the flap with a bunch or orcs hitting on you. The game might bug out with you not being able to move and not get the item either, resulting in death.The NM fight can be soloed at any level. Simply spawn the NMs with reraise on then die and wait for them to despawn, reraise then check the flaps without zoning.