That's an unverifiable statement and it would depend heavily on the shooters involved. It's as much conjecture as if I were to say that each bullet fired from each shooter's gun would only hit people without weapons.
Huh? I'm not sure how you can construct a plausible scenario where you're actually worse off with five armed citizens in the theater. Even if they each manage to accidentally shoot two additional people besides each other and even if the madman is half finished before one manages to take him down you still have many fewer people ending up getting shot.
