Only one of the 2 video cards are being used at any given time. It's more to extend the amount of time you can get in the battery per charge. When you're not in a graphically demanding program, it will use the Intel Integrated graphics (low power, and low performance). When you're running a graphically demanding program, the Nvidia GTX960M should kick in and take over from the Intel integrated graphics and start doing it's job (high power, high performance). The transition between Intel integrated to the discrete Nvidia graphics should be seamless.
What
@beers is saying is that if you want to plug in multiple external monitors to the laptop, you'll need to have 2 external monitors for a start. And on top of that, the 2 external monitors must have an hdmi connection and/or displayport. (It's probably best to leave the VGA one alone). Ideally the easiest is to have 1 that supports displayport, and the other supporting hdmi. If both only has one of the two outputs, you'll need to buy an adapter so it can plug into the laptop.