Zapo and Zepo are described quite similarly in Fernando Arrabal's play "Picnic in the Battlefield". For once, their uniforms are nearly identical except Zapo's uniform is a different color from Zepo, who fights in the enemy lines. Therefore, here we have an ironic way of telling us "birds of a feather": Same thing, different color.
Secondly, Zapo and Zepo's names are also nearly identical with the exception of a simple letter which, again, shows that it is the same thing with a different name: Two typical soldiers, sent to the battlefield, no clue as to what they are doing in war, none of them is sure why they are each other's enemies, they are about the same age, and have the same hopes and wishes. They both represent the ignorant youth sent to fight a bloody battle that they cannot even identify with.
We also find that Zapo and Zepo do similar activities, share similar likes, and Zapo's parents are quite taken by him, treating him like if he were their son as well by inviting him to the picnic. Again, both Zapo and Zepo represent the huge loss of young men that occurs as a result of war: Young men who often do not comprehend how dangerous war is, and who idealize it to an extent. Some of them hardly know what they are doing in the battlefield in the first place. All are just as innocent, just as unfairly taken, and just as naive. They are one same youth, with many different uniforms, names, and families.
You didnot include in the answer about the selection of Zapo and Zepo was also the exact same.