It is treated like normal armor. WT (weight) on the armor is stacked and compared to Strength. If total WT of all armors combined exceeds strength all agility /FS bonus to dodge and attack is lost for the character during combat.
WT for Armorfiend cloaks is 3, and WT for Armorfiend vests is 4
However if the players wants their character to take the hits more often but be better protected there are a few limited situations where it makes sense to overload on armor.
Now in the specific situation described above Nightmare Hide and Armorfiend Cloaks can be combined. They are both WT 3 armors giving a total armor WT of 6 so i would only recommend giving that combo to characters with a strength of 6 or higher for most normal mission conditions. (for NPCs like standard tribesmen their FS equals their strength stat)
The stats would be
AR: Nightmare Hide + Armorfiend Cloak (WT6)
vs Melee: 6
vs Missle: 12
vs Energy: 8
Vs Chill: 5
In the case of the Cloak of Unit 817 its a WT 2 armor so to combine it with Nightmare Hide (WT = 3) would be recommended for strength 5 or higher characters.
AR: Nightmare Hide + Cloak of Unit 817 (WT5)
vs Melee: 6
vs Missle: 6
vs Energy: 2 (12 with shield active)
Vs Chill: 5
(if an Olgog wears these armors you also add a +2 to all AR stats (melee, missile, energy, chill) due to their own natural hide underneath the manufactured armor)