It doesn't matter, it is all about positive and negative - excuse the pun.
So when Tsar asks Guard A to tell her what Guard B would say, she knows it will be the truth, no matter what. If Guard A were the liar, he would have to lie about what Guard B would have said. Since Guard B would have told the truth, Guard A would then have to tell the lie to Tsar. However, if Guard B were the liar, he would obviously have lied. However, Guard A would then have to answer the question truthfully which would be the lie from Guard B. Hence, no matter which guard is the liar and which guard is the truth-teller, the answer that Tsar gets would have one and exactly one lie in the answer. Knowing this, Tsar sits in the chair the Guard said would be the electric one.
I think this is correct - I am all confused now...