How long will those teams last?



Think of it in terms of the Prisoners Dilemma: both parties have two options, they can cooperate or they can lash out at each other. Cooperation has it's rewards for both parties (accomplish a goal, sharing resources, etc), but so does lashing out (steal the meat that the other guy is eating or his items, neutralizing a threat and so forth) especially when one side is cooperating since this means the rewards are likely higher and the risks of dying is lower in cases where one side cooperates (as opposed to situations which they both lash out, in which case you have a fight which can go either way). Humans are social animals predisposed towards cooperation and averse to lashing out against each other. The orcs you describe do not have said inhibitions. Every interaction between said orcs runs the risks of one side lashing out, especially since the Amoral Orcs are smart enough to know that their fellow orcs will lash out against them. This is a simplification of course, but there are other factors (such as instances in which there is a disparity of power between Orc A and Or B or when Orc A can kill or Orc B when Orc B is vulnerable allowing him to reap the rewards of murder without the risks that would usually come along with this).



Besides, some level of social codes were indicated in the books as mentioned.



As for WW1 remember that said conflict involved people who were conditioned to think of the other side as being an enemy of all they loved and whatever they're feelings once they got in the trenches, the other side was trying to kill them. Even so they would often do things like deliberately miss enemy soldiers when they shot.