If youre mathematically inclined, think the correct door is +1 and the incorrect door is -1. The one who tells the truth is +1, and the one who lies is -1. When you look at them, all you see is absolute value of 1, and you dont know if its positive or negative. By asking the question to 1 of them about how the other would answer, youre now ensuring you have 1 truth and 1 lie. A +1 and a -1. Now your question is multiplying -1 and +1, and it doesnt matter if you +1 x -1 or -1 x +1 because transitive property, you always get -1... or the door that is incorrect.