I never tried multitouch, but here is what I can guess from the documentation:
_input(event) callback, you can also get events of type
InputEventScreenTouch. You would know an event is of this type by doing:
if event.type == InputEvent.SCREEN_TOUCH:
# Do stuff with touch
These also contain an
telling you what is the ID of the touch. So for example, if you touch with one finger you will receive an event with index 0, then if you touch with a second finger you will get a second event with index 1, etc. So you can tell them apart when further events of this type occur.