To put it simply.<quoted text>This is an old chestnut. You ask computer 1 which door computer 2 would say is the deadly one and use it. If computer 1 is truthful, it will report the lie that computer 2 would tell. If computer 2 is the liar, it will lie about the truth that computer 1 would tell. Either way, you will get a lie from the responding computer and thus know the truth.
Two negatives equal a positive.
Two positives equal a positive.