You enter the first zombies aura, but are not in the other zombies aura yet. You take 5 damage
You keep moving, you hit the 2nd zombies aura. Does it deal 10 damage because it's cumulative with the 1st aura (meaning that you have now taken 15 cold damage)?
Absolutely not.
You take damage from the aura when you enter the aura (or begin your turn in the aura, but that's irrelevant for this discussion). So you move and enter the first zombie's aura. You take 5 damage. You continue your move and (presumably) do not leave the first zombie's aura, but enter the second zombie's aura. You take 5 damage from aura #2.
The cumulative just means that if the auras overlap you don't get to only take damage from one - at least, that's how I read it.