It would be a problem if you did shoot at exactly (down to the millisecond) the same time. However it is very unlikely that you'll pull the trigger during the same millisecond. More likely is the fact that it'll register quite a few milliseconds later. Act-Labs probably designed it with that fact in mind. As long as your shots are over ~34 milliseconds (the time for the electron gun to scan the screen once) apart, which is highly likely, then you are okay.