If you trust the guy telling you this, you can just ask him which door leads to certain doom. If not, you have no way of knowing if you're in a knights and knaves puzzle.
Am I missing something? If you ask brother A, he would say his brother likes small butts. If you ask brother B, he would also say his brother likes small butts. How do you differentiate?
It works the same as the original puzzle. If you ask the lying small butt brother, he'll lie and say his brother would say he likes small butts. If you ask the truthful big butt brother, he'd say his brother would say he likes big butts, because he knows his brother likes small butts and would lie about it.
Essentially the negatives work out so that each brother answers with the kind of butt they themselves like, which you can then use to determine which is truthful (though at this point that somehow seems less important).
I've never liked it when the Two Brothers riddles are worded like this. Either the "always telling the truth" brother is the one saying the riddle, and you know who to ask which door is safe or its the "always lying" brother, in which you don't know what the actual riddle is (as the one they just told you is a lie) and both doors probably lead to the certain death.
(I should probably add that I get the refrence in the post, I'm just being nitpicky)