The application of AIO just once is enough BUT many people, myself included many times, will (when time allows) apply a second layer.
I personally do not do so because I think the second layer adds anything but rather I do so thinking I may have missed an area or two. I lay down the first layer with the mindset it is cleaning.
I go over the plastic, nooks and crannies and buffer splatter. My second layer is put on with the mindset of polishing. I never use it as a stand alone product.
In the past I had the belief that many layers would benefit but have since concluded that that is incorrect.
Anthony