Ok, I'll bite too....
Even the instructions that come with the AIO suggest that: "If your car is *older...a year or more*, it may benefit from two applications of All-in-one. The first application does mostly cleaning with the second application laying down more acrylic protection. This second coat is optional."
So while KLASSE doesn't insist that two coats are nesessary, they do however recognise that a second coat can be beneficial in terms of protection.
Too, as others have mentioned, applying a second (or third etc) will ensure that all areas of the finish are covered. When applying a single coat, even the best of us can occasionally miss a spot, or apply too little product to be effective. It is very unlikely that you will miss the exact same area twice.
My own opinions are above. Worst case...layering 'only' replenishes areas of protection that have thinned and maintains the sealant coating at full strength.