FleshMasher is correct. That being said, on really laggy games I have noticed that RE doesn't except my inputs in time.
I personally think it uses the first input timing after the RE hits to dictate how much timing the remaining player input gets (i.e. if the person who selects A, B, K, G, etc... does so before the other person, it shortens how much time the other player has to select their input).
Haven't tested though. Just a theory based on a few whacky games!