So I'm writing a supernatural/fantasy story. The twist is that one of the main protagonists is hiding something that gives him an advantage in a fight. I won't go into details but it involves switching out with a twin in a fight. The secret is something only the main character knows but I can't really figure out how to go about hiding it without making it out of the blue. I don't want to make it too obvious but if I give no hints the twist is going to probably seem out of the blue. For example how would you react if a main character was in a fight was about to die and then his twin showed up stabbing the bad guy from behind? I feel like I should give hints like the bad guy saying things like "How did he get from here to there so fast?" or would that be too subtle? Essentially the twins mostly function as one character or is this too complicated? Any advice on how the main character should interact would also be appreciated.