If I had responded without reading other comments I would have said that the hotter, less dense "exhaust" gas from both candles — atmosphere enriched with carbon dioxide and soot and mostly depleted of oxygen — would accumulate at the top and snuff out the taller candle first. The idea that carbon dioxide is denser and therefore sinks is intriguing, but we do not observe a carpet of pure carbon dioxide on the ground in normal environments. Carbon dioxide molecules weigh, what, 12 + 16 + 16 units, so 44, and most of the atmosphere is diatomic nitrogen, which I think is a good bit lighter per molecule. (I am refraining from outside research.) The dynamic churning of fluids in motion maintains a pretty homogenous mixure, and only extreme gases like hydrogen and helium, or chlorine as in the Bhopal incident, would not be mixed in at all levels. My prediction: the top candle goes out first. I would expect it to burn longer than ten seconds, though. May I suggest a "quiz" tag? I have some ideas to contribute.