The problem with liar and truth riddles is this:

Let us assume the problem was one always says the truth and the other always says a lie then you can do the following easy trick. Ask one of them "Am I immortal?" if he will say "Yes" than he must be a lair. If he says "No

" he must be honest. I know this is a dirty move and the riddle is not supposed to be solved like that but it works .

I do not know if this strategy works here also, but there are a number of riddle where it does.