I will bring both players to a roof, and ask them to tell both stories. If the stories are different I will ask if one of them has proof of the situation. If one has proof, I will follow that proof. If they don't I will let them go because you never know, maybe player x shot y first, and therefore player y killed x. But tell them, even if they think they did nothing, that in the future they will follow the rules to prevent this situation next time.