2 People are shooting each other, neither of them are called out, and neither of them is a detective. Someone else decides to shoot one of the shooters because they're guessing they must be a T. Is there any rule on this cause I can't find one.
Shooting other players is traitorous, so you could kill them both. And then someone could kill you for shooting at other players. And that's how RDM trains are born.
You can kill both players for shooting at each other, however I'd recommend you wait until they kill one another.