DEATH AURA + INFERNAL LEGION

I tried to build a death aura necromancer with an infernal legion skitter bot. why two of them cannot deal damage simultaneously ?

Death Aura and Infernal Legion should deal damage simultaneously.

If you're asking about two skitterbots, Infernal Legion applies a debuff to surrounding enemies to deal damage. The same debuffs don't stack unless they are particularly designed to be stackable (poison).