I think it is more important to establish which door is which, than who likes which butts.
This can be achieved by asking
“Which door your brother point to if I ask him to point the door with the death?” The death will be behind the other door.
But if butts are more important for you, you can modify the question to ask about butts.
“How’re you guys doing? Like mentally? Sending this many people to their deaths must have weighed on you. It’s difficult to adjudicate borders, so you place the fate of others in the hands of something separate from you. Something guiltless. At night, beside a comforting fire and lush with drinks, I imagine that you reassure one another of your innocence. ‘The traveler didn’t die because we sent them to their death, the puzzle killed them.’ That’s a hard way to live and I’m sorry that you have to live it.”
*in unison* “I’m doing alright.” A tear rolls down the cheek of the brother on the left. You’ve got him.
Truth-telling brother bears none of the moral weight, of course. In his head he's never lied, never done anything wrong, it's everyone else who failed to trust him.