Ken,
I think the NTI scenario is similar to the one in this thread, but not identical. Here the question is what do you do if you've already decided to engage and see a second armed subject. I think the NTI scenario you described deals with the decision to engage.
Roadwild,
It is nearly impossible to train for the situation as originally described, where you've already decided to engage. It's a no win situation. If you don't shoot subject number 1 then he most likely will cause you great bodily harm or kill you, and if you do shoot subject number one and subject number two is a CCW holder attempting to intervene, then he's probably going to shoot you as soon as you shoot subject number one. In the situation as described, where a second armed subject shows up just as you're pressing the trigger on subject number one the only solution that gives you much of chance is to engage both of them. Because there is no way you could know what the second subject is up to in that split second.
Jeff