Each touch has a unique ID (which is an arbitrary number), and can be accessed using the TouchID expression in an event like On any touch start. The touch ID can then be stored in a variable and tracked using the XForID and YForID expressions. Finally comparing the TouchID in On any touch end indicates when that touch has been released.
So on touch start you can get the id for that touch..
Touch.TouchID is this id..
The touch zooming example and the touch rotating example shipped with Construct2 show examples of multitouch..