The most common actions only use one finger, they are "clicking" (or tapping as Apple call it) and "flicking" to scroll. The flick is very good at figuring out how much energy you put in to it so you have good control about just how fast you scroll.

The only current gesture that uses two fingers is the pinch/stretch gesture which you use to zoom in and out. You use it a lot while browsing the web. You can also use it on photos but you don't really need to very often ... except when giving demos of course, that's what everyone wants to see!

Most of the time it's just a very well put together single touch system that is very good at figuring out how hard you are touching as it were.

Bart.