Ok since Kro inspired me but this is my idea of those two singing....its not as prevalent as others but Lux (since I am bias) would be the dark haired man singing while Gil would be the back up since I don't know many duets then the Vampire knight one she had. But I believe in harmony Lux and Gil would sing this!! Enjoy!!! :D
It's from D.Gray-man and Gil is just super talented with a piano is all :3