Is there a way to resist a Poison?
Or if you are hit twice by a Poison effect you are automatically out?

Poison from Orcs and Spiders is inflicted as a result of a Called Shot which suggests a deep cut or puncture to me, in other words... a Piercing Blow.

So two Poison results is a KO and if the attack that introduces the second dose of Poison also reduces Endurance to zero... the character dies!

As for a way to resist it, the Protection check vs the Injury is the save I assume.

Under the Poisoned condition it says that "A Poisoned character is considered to be Wounded."
I still don't understand how it works... if you need to be wounded by a piercing attack, it means that the Poison would be the second wound.

The Poison is the Wound, I just doesn't heal through normal methods.