Millisecond Forums

Hello everyone!I designed an experiment, where participants should react to stimuli by releasing one out of two keys (here: "A" and "L"). My question is: How can I make sure, that both keys are pressed, before the stimulus is presented? I already created a message ("Please press the keys "A" and "L" now" = presskeyremind). I only want to measure the time from the presentation of the stimulus until the release of one of the keys.